drm/i915/skl+: calculate plane pixel rate (v4)

Don't use pipe pixel rate for plane pixel rate. Calculate plane pixel according
to formula

adjusted plane_pixel_rate = adjusted pipe_pixel_rate * downscale ammount

downscale amount = max[1, src_h/dst_h] * max[1, src_w/dst_w]
if 90/270 rotation use rotated width & height

v2: use intel_plane_state->visible instead of (fb == NULL) as per Matt's
    comment.

v3 (by Matt):
 - Keep downscale amount in 16.16 fixed point rather than converting to
   decimal fixed point.
 - Store adjusted plane pixel rate in plane state instead of the plane
   parameters structure that we no longer use.

v4 (by Matt):
 - Significant rebasing onto latest atomic watermark work
 - Don't bother storing plane pixel rate in state; just calculate it
   right before the calls that make use of it.
 - Fix downscale calculations to actually use width values when
   computing downscale_w rather than copy/pasted height values.

Cc: matthew.d.roper@intel.com
Signed-off-by: Kumar, Mahesh <mahesh1.kumar@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Kumar Mahesh <mahesh1.kumar@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463439121-28974-4-git-send-email-matthew.d.roper@intel.com