drm/i915: Skip aux plane stuff when there is no aux plane

when the hardware isn't going to use the aux plane there's no
real point in dealing with the relevant hardware restrictions.
So let's just skip all that when not necessary.

We can now also remove the offset=~0xfff behaviour for unused
color planes. Let's just zero out everyting so as to not leave
stale garbage behind to confuse people debugging the code.

v2: Explicitly set AUX_DIST to zero when there is no aux plane

Reviewed-by: Imre Deak <imre.deak@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201009120028.32422-1-ville.syrjala@linux.intel.com