drm/i915/fbdev: Select linear modifier explicitly

drm/i915/fbdev: Select linear modifier explicitly

Currently we use the implicit modifier fb creation path for fbdev,
but as we never call set_tiling on the bo it will always end up as
linear anyway. The rest of the code (eg. stride alignment) also
assumes that we'll use linear. Just select the linear modifier
explicitly.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20251003145734.7634-4-ville.syrjala@linux.intel.com