drm/amd/display: Drop unnecessary function call

After refactor our amdgpu_dm_atomic_commit, this function only invoke
drm_atomic_helper_commit. For this reason, this commit drops
amdgpu_dm_atomic_commit and add drm_atomic_helper_commit directly in the
atomic_commit hook.

v2: squash in warning fix (Alex)

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>