drm/amd/display: Fix low black values by increasing error

[WHY]
Regamma resolution for the first few black levels can have problems for
calibration.

[HOW]
HW LUT is divided into N power-of-2 regions each with K segments.  For
SDR mode we set min point at 2^-10 and increments of 2^-13. It's
generally more than 8-bit SDR needs, but some calibration tools and API
use 12-bit curves.
The fix shifts starting point to 2^-12 and starting increments at 2^-16.

Reviewed-by: Krunoslav Kovac <krunoslav.kovac@amd.com>
Signed-off-by: Peterson <peterson.guo@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>