drm: Extract drm_file.h

I'm torn on whether drm_minor really should be here or somewhere else.
Maybe with more clarity after untangling drmP.h more this is easier to
decide, for now I've put a FIXME comment right next to it. Right now
we need struct drm_minor for the inline drm_file type helpers, and so
it does kinda make sense to have them here.

Next patch will kerneldoc-ify the entire pile.

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170308141257.12119-10-daniel.vetter@ffwll.ch