r600g: modify block to only emit the first few dirty registers. (v2)

r600g: modify block to only emit the first few dirty registers. (v2)

This gets me from 2200 to 1978 dwords for a gears frame.

This is due to us having some 32-dwords blocks in the SPI, that we only
modify the first dwords off.

v2: fix dirty reg count from Bas Nieuwenhuizen

Signed-off-by: Dave Airlie <airlied@redhat.com>