drm/msm: update platform device API for DRM 4.14

The drm_device structure no longer contains a pointer to the
platform_device, however since the platform device contains
the drm_device structure some APIs needed refactoring.  This
change updates the ported SDE driver from 4.9 to work in 4.14.

Change-Id: Ie5e1cfae4daa1f9f9f5c5bf7916b1a32c6df755f
Signed-off-by: Steve Cohen <cohens@codeaurora.org>