drm/xe: make xe_ttm_funcs const

Place the function pointers in rodata. Also drop the extra declaration
while at it.

Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240117122044.1544174-1-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>