drm/xe: Update SPDX deprecated license identifier

drm/xe: Update SPDX deprecated license identifier

The "GPL-2.0" SPDX license identifier is deprecated. Update the
code to use "GPL-2.0-only" instead. Choose this identifier over
"GPL-2.0-or-later" since it's the most restrictive of the two and it's
not fully clear that "GPL-2.0" also allows "GPL-2.0-or-later".

Cc: Francois Dugast <francois.dugast@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231107082440.7568-1-thomas.hellstrom@linux.intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>