i965: Fix crash in do_blit_readpixels()
Fixes a crash in Khronos CTS packed_pixels tests. Cc: <mesa-stable@lists.freedesktop.org> Signed-off-by:Anuj Phogat <anuj.phogat@gmail.com> Reviewed-by:
Ian Romanick <ian.d.romanick@intel.com> (cherry picked from commit d714b20e)