drm: Fix color LUT rounding

drm: Fix color LUT rounding

The current implementation of drm_color_lut_extract()
generates weird results. Eg. if we go through all the
values for 16->8bpc conversion we see the following pattern:

in            out (count)
   0 -   7f ->  0 (128)
  80 -  17f ->  1 (256)
 180 -  27f ->  2 (256)
 280 -  37f ->  3 (256)
...
fb80 - fc7f -> fc (256)
fc80 - fd7f -> fd (256)
fd80 - fe7f -> fe (256)
fe80 - ffff -> ff (384)

So less values map to 0 and more values map 0xff, which
doesn't seem particularly great.

To get just the same number of input values to map to
the same output values we'd just need to drop the rounding
entrirely. But perhaps a better idea would be to follow the
OpenGL int<->float conversion rules, in which case we get
the following results:

in            out (count)
   0 -   80 ->  0 (129)
  81 -  181 ->  1 (257)
 182 -  282 ->  2 (257)
 283 -  383 ->  3 (257)
...
fc7c - fd7c -> fc (257)
fd7d - fe7d -> fd (257)
fe7e - ff7e -> fe (257)
ff7f - ffff -> ff (129)

Note that since the divisor is constant the compiler
is able to optimize away the integer division in most
cases. The only exception is the _ULL() case on 32bit
architectures since that gets emitted as inline asm
via do_div() and thus the compiler doesn't get to
optimize it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231013131402.24072-2-ville.syrjala@linux.intel.com
Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Maxime Ripard <mripard@kernel.org>