drm/bridge: imx: turn imx8{qm,qxp}-ldb into single-object modules

With the previous fix, these modules are built from a single C file.

Rename the source files so they match the module names.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Liu Ying <victor.liu@nxp.com>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230605120021.1774711-2-masahiroy@kernel.org