drm: remove duplicate check on parent and avoid BUG_ON

In drm_dev_init, parent is checked for NULL via assert after
checked in devm_drm_dev_init(). The patch removes the duplicate
check and replaces the assertion with WARN_ON. Further, it returns
-EINVAL consistent with the usage in devm_drm_dev_init.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191215194345.4679-1-pakki001@umn.edu