drm/amd/display: fix missing reg offset

[Why]
Initializing was missing reg offsets for the dmcub test debug registers
causing assert

[How]
Add initialization
Reviewed-by: Kazlauskas Nicholas <Nicholas.Kazlauskas@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dmub/src/dmub_dcn31.c

@@ -38,7 +38,10 @@
 
 const struct dmub_srv_dcn31_regs dmub_srv_dcn31_regs = {
 #define DMUB_SR(reg) REG_OFFSET_EXP(reg),
-	{ DMUB_DCN31_REGS() },
+	{
+		DMUB_DCN31_REGS()
+		DMCUB_INTERNAL_REGS()
+	},
 #undef DMUB_SR
 
 #define DMUB_SF(reg, field) FD_MASK(reg, field),