drm/amd/display: Fix potential null pointer dereference in dc_dmub_srv

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: Fix potential null pointer dereference in dc_dmub_srv Fixes potential null pointer dereference warnings in the dc_dmub_srv_cmd_list_queue_execute() and dc_dmub_srv_is_hw_pwr_up() functions. In both functions, the 'dc_dmub_srv' variable was being dereferenced before it was checked for null. This could lead to a null pointer dereference if 'dc_dmub_srv' is null. The fix is to check if 'dc_dmub_srv' is null before dereferencing it. Thus moving the null checks for 'dc_dmub_srv' to the beginning of the functions to ensure that 'dc_dmub_srv' is not null when it is dereferenced. Found by smatch & thus fixing the below: drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:133 dc_dmub_srv_cmd_list_queue_execute() warn: variable dereferenced before check 'dc_dmub_srv' (see line 128) drivers/gpu/drm/amd/amdgpu/../display/dc/dc_dmub_srv.c:1167 dc_dmub_srv_is_hw_pwr_up() warn: variable dereferenced before check 'dc_dmub_srv' (see line 1164)