mdp: various changes:
- comment out a commented for cycle in mdp_on (dunno why i commented it)
- use mdp_lut_enable() in mdp_preset_lut_update_lcdc instead
  hand made "mask off non LUT select bits", do the same thing
  but with spinlock.
- call mdp_lut_enable() at init so we can change LUT w/o issues