drm/amdgpu: Make struct drm_driver const

Make the definition of struct drm_driver
a constant, to follow the latest developments
in the DRM layer.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
[danvet: Rebase onto devm_drm_dev_alloc patch and drop the freesync
ioctl line again that escaped from internal trees.]
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20201104100425.1922351-6-daniel.vetter@ffwll.ch