drm/i915/gt: Prune 'inline' from execlists

Remove the extraneous inlines. The only split by the compiler that
looked dubious was execlists_schedule_out, so push the code around
slightly to move all the work into the out-of-line function.

In a normal build, bloat-o-meter shows that only the
execlists_schedule_out is contentious:

add/remove: 1/0 grow/shrink: 0/2 up/down: 803/-1532 (-729)
Function                                     old     new   delta
__execlists_schedule_out                       -     803    +803
execlists_submission_tasklet                6488    5766    -722
execlists_reset_csb.constprop               1587     777    -810
Total: Before=1605815, After=1605086, chg -0.05%

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210113151112.15212-1-chris@chris-wilson.co.uk