drm/i915: Clean up skl_plane_has_planar()

skl_plane_has_planar is hard to read, simplify the logic by checking for
support in the order of platform, pipe and plane.

No change in functionality intended.
v2: Fix logic for primary plane (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180827225624.4912-1-dhinakaran.pandiyan@intel.com