st/mesa: move st_manager_get_egl_image_surface into st_cb_eglimage.c

st/mesa: move st_manager_get_egl_image_surface into st_cb_eglimage.c

The only callers are here, and we will add generation of GL errors in
the following patch.  Rename the function to st_egl_image_get_surface,
pass the gl_context instead of st_context, and move the cast from
GLeglImageOES to void* into st_egl_image_get_surface.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>