drm/i915: Polish bdw_read_lut_10() a bit

Since bdw_read_lut_10() uses the auto-increment mode we must
have an equal number of entries in the software LUT and the
hardware LUT. WARN if that is not the case.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925131656.10022-9-ville.syrjala@linux.intel.com