intel: Rework intel_miptree_create_for_region() to wrap a BO.

intel: Rework intel_miptree_create_for_region() to wrap a BO.

I needed to do this for the PBO blit cases to use intel_miptree_blit().
But this also actually partially fixes a bug in EGLImage handling: We
can't share regions across contexts, because regions have a refcount that
isn't protected by a mutex, and different contexts can be simulataneously
accessed from multiple threads.  Now we just need to get regions out of
__DRIImage.  There was also a missing use of image->offset in the EGLImage
renderbuffer storage code.

Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>
Acked-by: Paul Berry <stereotype441@gmail.com>