Convert to using devm_drm_dev_alloc(),
as drm_dev_init() is going away.

v2: Remove drm_dev_put() since
    a) devres doesn't do refcounting, see
    Documentation/driver-api/driver-model/devres.rst,
    Section 4, paragraph 1; and since
    b) devres acts as garbage collector when
    the DRM device's parent's devres "action" callback
    is called to free the container device (amdgpu_device),
    which embeds the DRM dev.
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200918132505.2316382-4-daniel.vetter@ffwll.ch