drm/i915: Stop using mode->private_flags

Replace the use of mode->private_flags with a truly private bitmaks
in our own crtc state. We also need a copy in the crtc itself so the
vblank code can get at it. We already have scanline_offset in there
for a similar reason, as well as the vblank->hwmode which is assigned
via drm_calc_timestamping_constants(). Fortunately we now have a
nice place for doing the crtc_state->crtc copy in
intel_crtc_update_active_timings() which gets called both for
modesets and init/resume readout.

The one slightly iffy spot is the INHERITED flag which we want to
preserve until userspace/fb_helper does the first proper commit after
actually calling .detecti() on the connectors. Otherwise we don't have
the full sink capabilities (audio,infoframes,etc.) when .compute_config()
gets called and thus we will fail to enable those features when the
first userspace commit happens. The only internal commit we do prior to
that should be from intel_initial_commit() and there we can simply
preserve the INHERITED flag from the readout.

v2: Deal with INHERITED in sanitize_watermarks() as well

CC: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429103904.11727-1-ville.syrjala@linux.intel.com