drm/amd/display: Pass errors from amdgpu_dm_init() up

Errors in amdgpu_dm_init() are silently ignored and dm_hw_init()
will succeed. However often these are fatal errors and it would
be better to pass them up.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>