amdgpu: remove unreachable code

In the function amdgpu_uvd_cs_msg(), every branch in the switch
statement will have a return, so the code below the switch statement
will not be executed.

Eliminate the follow smatch warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c:845 amdgpu_uvd_cs_msg() warn:
ignoring unreachable code.
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_uvd.c

@@ -829,9 +829,8 @@ static int amdgpu_uvd_cs_msg(struct amdgpu_uvd_cs_ctx *ctx,
 
 	default:
 		DRM_ERROR("Illegal UVD message type (%d)!\n", msg_type);
-		return -EINVAL;
 	}
-	BUG();
+
 	return -EINVAL;
 }