drm/vkms: Extract vkms_config header

drm/vkms: Extract vkms_config header

Creating a new vkms_config structure will be more complex once we
start adding more options.

Extract the vkms_config structure to its own header and source files
and add functions to create and delete a vkms_config and to initialize
debugfs.

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/20250218101214.5790-5-jose.exposito89@gmail.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>