drm/omap: fix the handling of fb ref counts

With the recent primary-plane changes for drm, the primary plane's
framebuffer needs to be ref counted the same way as for
non-primary-planes. This was not done by the omapdrm driver, which
caused the ref count to drop to 0 too early, causing problems.

This patch moves the fb unref and ref from omap_plane_update to
omap_plane_mode_set. This way the fb refs are updated for both primary
and non-primary cases, as omap_plane_update calls omap_plane_mode_set.

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