i915: Fix map/unmap mismatches from leaving INTEL_FALLBACK during TNL.

The first rendering after context create didn't know of the color
buffer yet, triggering a sw fallback.  The intel_prepare_render() from
intelSpanRenderStart then found the buffer and turned off fallbacks,
but intelSpanRenderFinish was never called and things were left
mapped.  By checking buffers before making the call on whether to do
the fallback pipeline or not, we avoid the fallback change inside of
the rendering pipeline.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31561
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/dri/i915/intel_tris.c

@@ -1078,6 +1078,13 @@ intelRunPipeline(struct gl_context * ctx)
    if (ctx->NewState)
       _mesa_update_state_locked(ctx);
 
+   /* We need to get this done before we start the pipeline, or a
+    * change in the INTEL_FALLBACK() of its intel_draw_buffers() call
+    * while the pipeline is running will result in mismatched swrast
+    * map/unmaps, and later assertion failures.
+    */
+   intel_prepare_render(intel);
+
    if (intel->NewGLState) {
       if (intel->NewGLState & _NEW_TEXTURE) {
          intel->vtbl.update_texture_state(intel);
@@ -1092,7 +1099,9 @@ intelRunPipeline(struct gl_context * ctx)
    }
 
    intel_map_vertex_shader_textures(ctx);
+   intel->tnl_pipeline_running = true;
    _tnl_run_pipeline(ctx);
+   intel->tnl_pipeline_running = false;
    intel_unmap_vertex_shader_textures(ctx);
 
    _mesa_unlock_context_textures(ctx);
@@ -1228,6 +1237,8 @@ intelFallback(struct intel_context *intel, GLbitfield bit, GLboolean mode)
    if (mode) {
       intel->Fallback |= bit;
       if (oldfallback == 0) {
+	 assert(!intel->tnl_pipeline_running);
+
          intel_flush(ctx);
          if (INTEL_DEBUG & DEBUG_FALLBACKS)
             fprintf(stderr, "ENTER FALLBACK %x: %s\n",
@@ -1239,6 +1250,8 @@ intelFallback(struct intel_context *intel, GLbitfield bit, GLboolean mode)
    else {
       intel->Fallback &= ~bit;
       if (oldfallback == bit) {
+	 assert(!intel->tnl_pipeline_running);
+
          _swrast_flush(ctx);
          if (INTEL_DEBUG & DEBUG_FALLBACKS)
             fprintf(stderr, "LEAVE FALLBACK %s\n", getFallbackString(bit));

src/mesa/drivers/dri/intel/intel_context.h

@@ -199,6 +199,7 @@ struct intel_context
    drm_intel_bo *first_post_swapbuffers_batch;
    GLboolean need_throttle;
    GLboolean no_batch_wrap;
+   bool tnl_pipeline_running; /**< Set while i915's _tnl_run_pipeline. */
 
    struct
    {