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>

src/gallium/drivers/radeonsi/si_shader.h

@@ -502,7 +502,7 @@ union si_shader_part_key {
 		unsigned	ancillary_vgpr_index:5;
 		unsigned	wqm:1;
 		char		color_attr_index[2];
-		char		color_interp_vgpr_index[2]; /* -1 == constant */
+		signed char	color_interp_vgpr_index[2]; /* -1 == constant */
 	} ps_prolog;
 	struct {
 		struct si_ps_epilog_bits states;