drm/i915/gen9: don't call ilk_pipe_pixel_rate() twice on the same function

We used to call skl_pipe_pixel_rate(), which used to be a single
one-line return, but now we're  calling ilk_pipe_pixel_rate() which is
not as simple, so it's better to just call it once and store the
computed value for reuse.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1475872138-16194-2-git-send-email-paulo.r.zanoni@intel.com