drm/i915: Move the DSB->mmio fallback into the LUT code

drm/i915: Move the DSB->mmio fallback into the LUT code

The use of DSB has to be done differently on a case by case basis.
So no way this kind of blind mmio fallback in the guts of the DSB
code will work properly. Move it at least one level up into the
LUT loading code. Not sure if this is the way we want do the
DSB vs. mmio handling in the end, but at least it's a bit
closer than what we had before.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123152638.20622-8-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>