drm: Extract drm_mode_object.[hc]

Just for the struct drm_mode_object base class. The header file was
already partially extracted to help untangle the include loops.

v2:
- Also move the generic get/set property ioctls. At first this seemed
  like a bad idea since it requires making drm_mode_crtc_set_obj_prop
  non-static. But eventually that will get split away too (like
  the connector version already is) for both crtc and planes. Hence I
  reconsidered.

- drm_mode_object.[hc] instead of drm_modeset.[hc], which requires
  renaming the drm_modeset.h header I already started building up.
  This is more consistent (matches the name of the main structure),
  and I want to be able to use drm_modeset.[hc] for the basic modeset
  init/cleanup functionality like drm_mode_config_init.

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160829082757.17913-3-daniel.vetter@ffwll.ch