drm/i915/dsb: Nuke the MMIO->indexed register write logic

drm/i915/dsb: Nuke the MMIO->indexed register write logic

We've determined that indexed DSB writes are only faster
than MMIO writes when writing the same register ~5 or more
times. That seems very unlikely to happen in any other case
than when using indexed LUT registers. Simplify the code
by removing the MMIO->indexed write conversion logic and
just emit the instruction as an indexed write from the get go.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241120164123.12706-4-ville.syrjala@linux.intel.com
Reviewed-by: Uma Shankar <uma.shankar@intel.com>