intel: in intel_context struct use typedef for sarea struct

Using drm_i915_sarea_t instead of struct drm_i915_sarea seems to be
a common standard now, therefore fix it also in intel_context
structure. Additionally this silences a compiler warning:

intel_swapbuffers.c: In function `intelFixupVblank':
intel_swapbuffers.c:48: warning: initialization from incompatible pointer type

Signed-off-by: Tobias Doerffel <tobias.doerffel@gmail.com>

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

@@ -312,7 +312,7 @@ struct intel_context
    __DRIdrawablePrivate *driReadDrawable;
    __DRIscreenPrivate *driScreen;
    intelScreenPrivate *intelScreen;
-   volatile struct drm_i915_sarea *sarea;
+   volatile drm_i915_sarea_t *sarea;
 
    GLuint lastStamp;