drm/i915: Nuke plane->can_scale/min_downscale

We can easily calculate the plane can_scale/min_downscale on demand.
And later on we'll probably want to start calculating these dynamically
based on the cdclk just as skl already does.

Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180907152413.15761-9-ville.syrjala@linux.intel.com