drm/i915: Replace I915_MODE_FLAG_INHERITED with a boolean

There's no reason for I915_MODE_FLAG_INHERITED to exist as a flag
anymore. Just make it a boolean.

v2: Deal with sanitize_watermarks()

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/20200429103936.11850-1-ville.syrjala@linux.intel.com