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)