drm/omap: fix omap_gem_put_paddr() error handling

If tiler_unpin() call in omap_gem_put_paddr() fails,
omap_gem_put_paddr() will immediately stop processing and return an
error.

This patch remoes that error checking, and also removes
omap_gem_put_paddr()'s return value, because:

 * The caller of omap_gem_put_paddr() can do nothing if an error
   happens, so it's pointless to return an error value

 * If tiler_unpin() fails, the GEM object will possibly be left in an
   undefined state, where the DMM mapping may have been removed, but the
   GEM object still thinks everything is as it should be, leading to
   crashes later.

 * There's no point in returning an error from a "free" call, as the
   caller can do nothing about it. So it's better to clean up as much as
   possible.

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