freedreno/a4xx: rasterizer_discard support

freedreno/a4xx: rasterizer_discard support

This one is slightly annoying, since trying to write RBRC from draw
would clobber values set in the tiling/gmem code.  We could do command-
stream patching for RBRC, as is done on a3xx.  Although since it seems
to be a rarely used feature, it is easier just to do RMW to set/clear
the bit.

Fixes dEQP-GLES3.functional.rasterizer_discard.basic.write_depth_triangles
and related tests.

a3xx still needs the same feature, although there it probably makes more
sense to take advantage of the existing cmdstream patching which is
required for RBRC for other reasons.

Signed-off-by: Rob Clark <robclark@freedesktop.org>