drm/radeon/kms: add missing safe regs for 6xx/7xx

needed for HiS in mesa.
Signed-off-by: NAlex Deucher <alexdeucher@gmail.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/reg_srcs/r600

@@ -708,6 +708,7 @@ r600 0x9400
 0x00028D0C DB_RENDER_CONTROL
 0x00028D10 DB_RENDER_OVERRIDE
 0x0002880C DB_SHADER_CONTROL
+0x00028D28 DB_SRESULTS_COMPARE_STATE0
 0x00028D2C DB_SRESULTS_COMPARE_STATE1
 0x00028430 DB_STENCILREFMASK
 0x00028434 DB_STENCILREFMASK_BF