drm/i915: Decouple SKL stride units from intel_fb_stride_alignment()

In the future framebuffer stride alignment requirements won't exactly
match the units in which skl+ plane stride is specified. So extract
the code for the skl+ stuff into a separate helper.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180925193714.25280-3-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>