i965: send all warnings through _mesa_warning()

One warning message:
   drm_i915_getparam: -22

was still being sent to fprintf().  This causes all Piglit tests to fail,
even with MESA_DEBUG=0.

Using _mesa_warning() to emit the message allows the general Mesa controls
for messages like this to be applied.

src/mesa/drivers/dri/intel/intel_screen.c

@@ -236,7 +236,7 @@ intel_get_param(__DRIscreenPrivate *psp, int param, int *value)
 
    ret = drmCommandWriteRead(psp->fd, DRM_I915_GETPARAM, &gp, sizeof(gp));
    if (ret) {
-      fprintf(stderr, "drm_i915_getparam: %d\n", ret);
+      _mesa_warning(NULL, "drm_i915_getparam: %d\n", ret);
       return GL_FALSE;
    }