drm/xe: Fix exporting xe buffers multiple times

drm/xe: Fix exporting xe buffers multiple times

The `struct ttm_resource->placement` contains TTM_PL_FLAG_* flags, but
it was incorrectly tested for XE_PL_* flags.
This caused xe_dma_buf_pin() to always fail when invoked for
the second time. Fix this by checking the `mem_type` field instead.

Fixes: 7764222d54b7 ("drm/xe: Disallow pinning dma-bufs in VRAM")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: intel-xe@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.8+
Signed-off-by: Tomasz Rusinowicz <tomasz.rusinowicz@intel.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250218100353.2137964-1-jacek.lawrynowicz@linux.intel.com
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>