drm/fdinfo: Protect against driver unbind

If we unbind a driver from the PCI device with an active DRM client,
subsequent read of the fdinfo data associated with the file descriptor in
question will not end well.

Protect the path with a drm_dev_enter/exit() pair.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Fixes: 3f09a0cd ("drm: Add common fdinfo helper")
Cc: <stable@vger.kernel.org> # v6.5+
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250418162512.72324-1-tvrtko.ursulin@igalia.com