drm/i915: Clean up pre-skl wm calling convention

drm/i915: Clean up pre-skl wm calling convention

Just pass the full atomic state+crtc to the pre-skl watermark
functions, and clean up the types/variable names around the area.

Note that having both .compute_pipe_wm() and .compute_intermediate_wm()
is entirely redundant now. We could unify them to a single vfunc.
But let's do this one step at a time.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609085632.22026-5-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>