drm/i915: Use intel state as much as possible in wm code

Instead of directly referencing drm_crtc_state, convert to
intel_ctc_state and use the base struct. This is useful when we're
making the split between uapi and hw state, and also makes the
code slightly more readable.

A lot of places also use cstate, instead of the more common crtc_state.
Clean those up to use crtc_state. Same for pstate vs plane_state. (Ville)

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190628085517.31886-7-maarten.lankhorst@linux.intel.com