drm/vgem: Fix vgem_init to get drm device available.

Modify vgem_init to take platform dev as parent in drm_dev_init.
This will make drm device available at "/sys/devices/platform/vgem"
in x86 chromebook.

v2: rebase, address checkpatch typo and line over 80 characters

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Deepak Sharma <deepak.sharma@amd.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181023163550.15211-1-emil.l.velikov@gmail.com