drm/shmem: Cleanup drm_gem_shmem_create_with_handle()

Once we create the handle, the handle owns the reference.  Currently
nothing was doing anything with the shmem ptr after the handle was
created, but let's change drm_gem_shmem_create_with_handle() to not
return the pointer, so-as to not encourage problematic use of this
function in the future.  As a bonus, it makes the code a bit cleaner.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230123154831.3191821-1-robdclark@gmail.com