drm/omap: Fix locking in omap_gem_new_dmabuf()

drm/omap: Fix locking in omap_gem_new_dmabuf()

omap_gem_new_dmabuf() creates the new gem object, and then takes and
holds the omap_obj->lock for the rest of the function. This has two
issues:

- omap_gem_free_object(), which is called in the error paths, also takes
  the same lock, leading to deadlock
- Even if the above wouldn't happen, in the error cases
  omap_gem_new_dmabuf() still unlocks omap_obj->lock, even after the
  omap_obj has already been freed.

Furthermore, I don't think there's any reason to take the lock at all,
as the object was just created and not yet shared with anyone else.

To fix all this, drop taking the lock.

Fixes: 3cbd0c587b12 ("drm/omap: gem: Replace struct_mutex usage with omap_obj private lock")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/511b99d7-aade-4f92-bd3e-63163a13d617@stanley.mountain/
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240806-omapdrm-misc-fixes-v1-3-15d31aea0831@ideasonboard.com