drm/omap: accept NULL for dma_addr in omap_gem_pin

Allow NULL to be passed in 'dma_addr' for omap_gem_pin(), in case the
caller does not need the dma_addr.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010120000.1421-4-jjhiblot@ti.com