drm/i915: Eliminate PIPECONF RMWs from .color_commit()

Eliminate the PIPECONF RMWs from .comit_commit() so
that we can finally declare the whole vblank evade part
(and the noarm() part) of the pipe commit free of register
reads. Or at least I hope that's the last read...

Only the i9xx/ilk codepaths need this for now, but let's
add the same thing for hsw+ just in case we want to start
calling that during fastsets at some point (eg. to change
dithering settings/etc.).

Should open up the way to start experimenting with
different DSB usage approaches for pipe commits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220413192607.27533-1-ville.syrjala@linux.intel.com


Reviewed-by: Uma Shankar <uma.shankar@intel.com>