1. Remove DPMS logic. As stated in KMS kernel docs: CRTC "activate"
   property was implemented as a simplified atomic replacement of DPMS.
   And kernel internally will just update "activate" on DPMS setting.

2. Add SetDisplayActivate(bool state) method to compositor class,
   which is now replacement for SetDpmsMode().

3. Move mode settings out of DrmComposition class to DrmCompositor class.
   From now on DrmComposition describes only layer-to-plane composition
   as it should be.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>