drm: forward declare struct drm_file in drm_syncobj.h

drm_syncobj.h uses struct drm_file pointers, forward declare struct
drm_file to make the header self-contained. This prepares for dropping
drmP.h from files including drm_syncobj.h.

Switch from "" to <> includes while at it.

v2: forward declare instead of including drm_file.h (Daniel)

[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/20181228082815.10797-1-jani.nikula@intel.com