drm/amdgpu/uvd4.2: Don't initialize UVD 4.2 when DPM is disabled

drm/amdgpu/uvd4.2: Don't initialize UVD 4.2 when DPM is disabled

UVD 4.2 doesn't work at all when DPM is disabled because
the SMU is responsible for ungating it. So, Linux fails
to boot with CIK GPUs when using the amdgpu.dpm=0 parameter.

Fix this by returning -ENOENT from uvd_v4_2_early_init()
when amdgpu_dpm isn't enabled.

Note: amdgpu.dpm=0 is often suggested as a workaround
for issues and is useful for debugging.

Fixes: a2e73f56fa62 ("drm/amdgpu: Add support for CIK parts")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>