radv: Do not set SX DISABLE bits for RB+ with unused surfaces.

radv: Do not set SX DISABLE bits for RB+ with unused surfaces.

The extra bits in CB_SHADER_MASK break dual source blending in
SkQP on a Stoney device. However:

- As far as I can tell, some other dual source blend tests are passing
  before and after the change.
- A hacked around skqp passes on my Vega desktop and Raven laptop
- Getting Skqp to give any useful info or to run it outside of Android
  on ChromeOS is proving difficult.

I have confirmed 3 strategies that seem to work:
- The old radv behavior of setting CB_SHADER_MASK to 0xF
- AMDVLK: CB_SHADER_MASK = 0xFF, and the 3 RB+ regs
  are 0.
- radeonsi: CB_SHADER_MASK = 0xFF, but does not set DISABLE
  bits in SX_BLEND_OPT_CONTROL for CB 1-7.

Let us use the radeonsi solution as that solution also seems like the correct
thing to do for holes. I have tested on my Raven laptop that setting the high
surfaces to not disabled and downconvert to 32_R does not imply a performance
penalty.

Fixes: e9316fdfd48 "radv: fix setting CB_SHADER_MASK for dual source blending"
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3670>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3670>