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)

Fixes: 028bac58 ("drm/amd/display: decouple dmcub execution to reduce lock granularity")
Fixes: 65138eb7

 ("drm/amd/display: Add DCN35 DMUB")
Cc: JinZe.Xu <jinze.xu@amd.com>
Cc: Hersen Wu <hersenxs.wu@amd.com>
Cc: Josip Pavic <josip.pavic@amd.com>
Cc: Roman Li <roman.li@amd.com>
Cc: Qingqing Zhuo <Qingqing.Zhuo@amd.com>
Cc: Harry Wentland <Harry.Wentland@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>