intel: fix mapping of malloc'd renderbuffers
intel: fix mapping of malloc'd renderbuffers

This fixes accum buffer operations.  The accumulation buffer is the
only malloc-based renderbuffer for the intel drivers.

v2: apply x/y offset to returned pointer

NOTE: This is a candidate for the 8.0 branch.

Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

benaadams/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,mcanthony/glsl-optimizer,mcanthony/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,mcanthony/glsl-optimizer,wolf96/glsl-optimizer,jbarczak/glsl-optimizer,zeux/glsl-optimizer,djreep81/glsl-optimizer,jbarczak/glsl-optimizer,mapbox/glsl-optimizer,benaadams/glsl-optimizer,bkaradzic/glsl-optimizer,benaadams/glsl-optimizer,jbarczak/glsl-optimizer,benaadams/glsl-optimizer,zeux/glsl-optimizer,bkaradzic/glsl-optimizer,dellis1972/glsl-optimizer,zz85/glsl-optimizer,bkaradzic/glsl-optimizer,tokyovigilante/glsl-optimizer,wolf96/glsl-optimizer,mcanthony/glsl-optimizer,zz85/glsl-optimizer,mcanthony/glsl-optimizer,jbarczak/glsl-optimizer,tokyovigilante/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,benaadams/glsl-optimizer,wolf96/glsl-optimizer,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,jbarczak/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,dellis1972/glsl-optimizer,mapbox/glsl-optimizer,dellis1972/glsl-optimizer,mapbox/glsl-optimizer,zz85/glsl-optimizer,mapbox/glsl-optimizer,djreep81/glsl-optimizer,djreep81/glsl-optimizer,wolf96/glsl-optimizer,tokyovigilante/glsl-optimizer,zz85/glsl-optimizer,zeux/glsl-optimizer,wolf96/glsl-optimizer,dellis1972/glsl-optimizer,metora/MesaGLSLCompiler,dellis1972/glsl-optimizer,tokyovigilante/glsl-optimizer