drm: Nuke mode->private_flags

drm: Nuke mode->private_flags

The last two uses of mode->private_flags (in i915 and gma500)
are now gone. So let's remove mode->private_flags entirely.

v2: Drop the earlier int->u8 conversion

CC: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200428171940.19552-16-ville.syrjala@linux.intel.com
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>