drm/v3d: Use devm_drm_dev_alloc

Also allows us to simplify the unroll code since the drm_dev_put
disappears.

Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-9-daniel.vetter@ffwll.ch