drm/msm: Drop load/unload drm_driver ops

The load/unload drm_driver ops are deprecated. They should be removed as
they result in creation of devices visible to userspace even before
the drm_device is registered.

Drop these ops and use drm_dev_alloc/register and drm_dev_unregister/unref
to explicitly create and destroy the drm device in the msm platform
driver's bind and unbind ops. With this in use, the drm connectors are
only registered once the drm_device is registered.

It also fixes the issue of stray debugfs files after the msm module is
removed. With this, all the debugfs files are removed, and allows
successive module insertions/removals.

Signed-off-by: Archit Taneja <architt@codeaurora.org>