drm/omap: gem: Merge __omap_gem_get_pages() and omap_gem_attach_pages()

The __omap_gem_get_pages() function is a wrapper around
omap_gem_attach_pages() that returns the omap_obj->pages pointer through
a function argument. Some callers don't need the pages pointer, and all
of them can access omap_obj->pages directly. To simplify the code merge
the __omap_gem_get_pages() wrapper with omap_gem_attach_pages() and
update the callers accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>