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>

Author: ckborah

drivers/gpu/drm/i915/display/intel_color.c

@@ -613,6 +613,8 @@ static u16 ctm_to_twos_complement(u64 coeff, int int_bits, int frac_bits)
 	if (CTM_COEFF_NEGATIVE(coeff))
 		c = -c;
 
+	int_bits = max(int_bits, 1);
+
 	c = clamp(c, -(s64)BIT(int_bits + frac_bits - 1),
 		  (s64)(BIT(int_bits + frac_bits - 1) - 1));