drm/i915: Generalize skl_ddb_allocation_overlaps()

drm/i915: Generalize skl_ddb_allocation_overlaps()

Make skl_ddb_allocation_overlaps() useful for other callers
besides skl_update_crtcs(). We'll need it to do plane updates
as well.

And while we're here we can reduce the stack utilization a
bit by noting that each struct skl_ddb_entry is 4 bytes whereas
a pointer to one is 8 bytes (on 64bit). So we'll switch to an
array of structs from the array of pointers we used before.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181101150605.18235-9-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>