drm/i915: Use the correct format string modifier for ptrdiff_t

When compiling on 32bits, I have the following warning:

drivers/gpu/drm/i915/i915_cmd_parser.c:405:4: warning: format ‘%ld’
expects argument of type ‘long int’, but argument 7 has type ‘int’
[-Wformat=]
    DRM_DEBUG_DRIVER("CMD: Command length exceeds batch length: 0x%08X
    length=%d batchlen=%ld\n",

The ptrdiff_t type has its own modifier: 't'.

Cc: Brad Volkin <bradley.d.volkin@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Brad Volkin <bradley.d.volkin@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>