drm/amd/display: Fix possible NULL dereference on device remove/driver unload

As part of a cleanup amdgpu_dm_fini() function, which is typically
called when a device is being shut down or a driver is being unloaded

The below error message suggests that there is a potential null pointer
dereference issue with adev->dm.dc.

In the below, line of code where adev->dm.dc is used without a preceding
null check:

for (i = 0; i < adev->dm.dc->caps.max_links; i++) {

To fix this issue, add a null check for adev->dm.dc before this line.

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm.c:1959 amdgpu_dm_fini() error: we previously assumed 'adev->dm.dc' could be null (see line 1943)

Fixes: 006c26a0 ("drm/amd/display: Fix crash on device remove/driver unload")
Cc: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Roman Li <roman.li@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>