drm/i915/color: Preserve sign bit when int_bits is Zero

drm/i915/color: Preserve sign bit when int_bits is Zero

When int_bits == 0, we lose the sign bit when we do the range check
and apply the mask.

Fix this by ensuring a minimum of one integer bit, which guarantees space
for the sign bit in fully fractional representations (e.g. S0.12)

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patch.msgid.link/20251203085211.3663374-7-uma.shankar@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>