drm/i915/gt: Prune inlines

Remove all the manual inlines from non-critical sections in gt/

add/remove: 2/0 grow/shrink: 0/3 up/down: 762/-1473 (-711)
Function                                     old     new   delta
mi_set_context.isra                            -     602    +602
write_dma_entry                                -     160    +160
__set_pd_entry                               214      69    -145
clear_pd_entry                               190      42    -148
ring_request_alloc                          2021     841   -1180
Total: Before=1605086, After=1604375, chg -0.04%

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210113152224.29794-1-chris@chris-wilson.co.uk