drm: remove allow_fb_modifiers

The allow_fb_modifiers flag is unnecessary since it has been replaced
with fb_modifiers_not_supported flag.

v3:
 - change the order as follows:
   1. add fb_modifiers_not_supported flag
   2. add default modifiers
   3. remove allow_fb_modifiers flag

v5:
 - keep a sanity check in plane init func

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128060836.11216-4-etom@igel.co.jp