intel: Refactor intel_update_renderbuffers()

intel: Refactor intel_update_renderbuffers()

Extract the code that queries DRI2 to obtain the DRIdrawable's buffers
into intel_query_dri2_buffers_no_separate_stencil().

Extract the code that assigns the DRI buffer's DRM region to the
corresponding renderbuffer into
intel_process_dri2_buffer_no_separate_stencil().

Rationale
---------
The next commit enables intel_update_renderbuffers() to query for separate
stencil and hiz buffers. Without separating the separate-stencil and
no-separate-stencil paths, intel_update_renderbuffers() degenerates into
an impenetrable labyrinth of if-trees.

CC: Ian Romanick <idr@freedesktop.org>
CC: Kristian Høgsberg <krh@bitplanet.net>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>