modesetting: use drmmode_bo_import() for rotate_fb

drmmode_shadow_allocate() still uses drmModeAddFB() which may fail if
the format is not as expected, preventing from using a rotated output.

Change it to use the new function drmmode_bo_import() which takes care
of calling the drmModeAddFB2() API.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106715


Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Tested-by: Tomas Pelka <tpelka@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>