drm: omapdrm: simplify omap_gem_pin

Move tiler related code to its own function.

Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1642587791-13222-2-git-send-email-ivo.g.dimitrov.75@gmail.com