video: adf: Fix modular build

Builds with ADF configured as module fail the following errors.

ERROR: "adf_fops" [drivers/video/adf/adf_sysfs.ko] undefined!
ERROR: "adf_obj_sysfs_find" [drivers/video/adf/adf_fops.ko] undefined!
ERROR: "adf_buffer_cleanup" [drivers/video/adf/adf_fops.ko] undefined!
ERROR: "adf_attachment_validate" [drivers/video/adf/adf_client.ko] undefined!
ERROR: "adf_attachment_find" [drivers/video/adf/adf_client.ko] undefined!
ERROR: "adf_buffer_mapping_cleanup" [drivers/video/adf/adf_client.ko] undefined!
ERROR: "adf_attachment_free" [drivers/video/adf/adf_client.ko] undefined!
ERROR: "adf_obj_find_event_refcount" [drivers/video/adf/adf_client.ko] undefined!
ERROR: "adf_file_queue_event" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_interface_sysfs_init" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_interface_sysfs_destroy" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_device_sysfs_init" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_device_sysfs_destroy" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_sysfs_destroy" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_overlay_engine_sysfs_init" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_overlay_engine_sysfs_destroy" [drivers/video/adf/adf.ko] undefined!
ERROR: "adf_sysfs_init" [drivers/video/adf/adf.ko] undefined!

If ADF is configured as module, each of the object files ends up being
a separate module. Since the functions are used across the various files
but not exported, this results in the observed build errors.
Modify the Makefile to create a single module instead.

Fixes: 066a50cee536 ("video: add atomic display framework")
Signed-off-by: Guenter Roeck <groeck@chromium.org>
(cherry picked from commit bb485f9d7aac6f4f2a75e0c3bd48d3b55966afb6)
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>