drm/i915: Allow downscale factor of <3.0 on glk+ for all formats

Bspec says that glk+ max downscale factor is <3.0 for all pixel formats.
Older platforms had a max of <2.0 for NV12. Update the code to deal with
this.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190913193157.9556-3-ville.syrjala@linux.intel.com