drm/i915: Use intel_ types more consistently for watermark code (v2)

Try to be more consistent about intel_* types rather than drm_* types
for lower-level driver functions.

v2:
 - Also drop the intel_crtc parameter from compute_intermediate_wm()
   since we can just extract it from the crtc_state parameter. (Ville)

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181210215415.19854-1-matthew.d.roper@intel.com