radeonsi: Use signed char for color_interp_vgpr_index

radeonsi: Use signed char for color_interp_vgpr_index

color_interp_vgpr_index was declared as a generic char value.
Because signed values are used in this variable, the result
was not safe across architectures and crashed on ppc64[el]
and arm.

Declare color_interp_vgpr_index as a signed type.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>