drm/komeda: Add komeda_crtc_prepare/unprepare

These two function will be used by komeda_crtc_enable/disable to do some
prepartion works when enable/disable a crtc. like enable a crtc:
  1. Adjust display operation mode.
  2. Enable/prepare needed clk.

v2: Rebase

Signed-off-by: James Qian Wang (Arm Technology China) <james.qian.wang@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>