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>