drm/vkms: Move default_config creation to its own function

drm/vkms: Move default_config creation to its own function

Extract the initialization of the default configuration to a function.
Refactor, no functional changes.

Reviewed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Co-developed-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250218101214.5790-6-jose.exposito89@gmail.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>