drm/panthor: Add FOP_UNSIGNED_OFFSET to fop_flags

Since commit 641bb439 ("fs: move FMODE_UNSIGNED_OFFSET to fop_flags")
the FMODE_UNSIGNED_OFFSET flag has been moved to fop_flags and renamed,
but the patch failed to make the changes for the panthor driver.
When user space opens the render node the WARN() added by the patch
gets triggered.

Fixes: 641bb439 ("fs: move FMODE_UNSIGNED_OFFSET to fop_flags")
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240920102802.2483367-1-liviu.dudau@arm.com