drm: include idr.h from drm_file.h

drm_file.h embeds idr structures in DRM-specific structures. Include the
corresponding header to make drm_file.h self-contained. Make it easier
to drop drmP.h includes.

[Updated commit message per Laurent's review while applying.]

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/2818b15332ab562722dfc324cf977b7eb4a04401.1545915059.git.jani.nikula@intel.com