drm/i915: Return the mask of enabled infoframes from ->inforame_enabled()

We want to start tracking which infoframes are enabled, so let's replace
the boolean flag with a bitmask.

We'll abstract the bitmask so that it's not platform dependent. That
will allow us to examine the bitmask later in platform independent code.

v2: Don't map VIDEO_DIP_ENABLE to the null packet (Daniel)
    Put a FIXME in the lspcon function

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190225174106.2163-3-ville.syrjala@linux.intel.com