drm/radeon: add error output if VM CS fails on cayman

So we know why the CS was rejected.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/evergreen_cs.c

@@ -2829,6 +2829,7 @@ static bool evergreen_vm_reg_valid(u32 reg)
 	case CAYMAN_SQ_EX_ALLOC_TABLE_SLOTS:
 		return true;
 	default:
+		DRM_ERROR("Invalid register 0x%x in CS\n", reg);
 		return false;
 	}
 }