loader: make image_format_to_fourcc() non-static

loader: make image_format_to_fourcc() non-static

the image_format_to_fourcc() function will be used from
egl/wayland hence make it non-static. Also move the function
into loader_dri_helper.c from loader_dri3_helper.c since
loader_dri3_helper library depends on xcb which will make
egl wayland depend on xcb indirectly.

v2: add loader tag to extern image_format_to_fourcc() (Marek Olšák)

V3: move image_format_to_fourcc to loader_dri_helper.c
Signed-off-by: Yogesh Mohanmarimuthu <yogesh.mohanmarimuthu@amd.com>
Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13422>