drm/amd/display: Fix a NULL pointer dereference in amdgpu_dm_connector_add_common_modes()

In the Linux kernel, the following vulnerability has been resolved:

drm/amd/display: Fix a NULL pointer dereference in amdgpu_dm_connector_add_common_modes()

In amdgpu_dm_connector_add_common_modes(), amdgpu_dm_create_common_mode()
is assigned to mode and is passed to drm_mode_probed_add() directly after
that. drm_mode_probed_add() passes &mode->head to list_add_tail(), and
there is a dereference of it in list_add_tail() without recoveries, which
could lead to NULL pointer dereference on failure of
amdgpu_dm_create_common_mode().

Fix this by adding a NULL check of mode.

This bug was found by a static analyzer.

Builds with 'make allyesconfig' show no new warnings,
and our static analyzer no longer warns about this code.