vfio-mdev: Remove flag FBINFO_DEFAULT from fbdev sample driver

The flag FBINFO_DEFAULT is 0 and has no effect, as struct fbinfo.flags
has been allocated to zero by framebuffer_alloc(). So do not set it.

Flags should signal differences from the default values. After cleaning
up all occurrences of FBINFO_DEFAULT, the token will be removed.

v2:
	* fix commit message (Miguel)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Kirti Wankhede <kwankhede@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230715185343.7193-8-tzimmermann@suse.de