drm/omap: clear omap_obj->paddr in omap_gem_put_paddr()

Clear omap_obj's paddr when unmapping the memory, so that it's easier to
catch bad use of the paddr.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

diff --cc drivers/gpu/drm/omapdrm/omap_gem.c
Simple merge