drm/msm: Improve drm/sched point of no return rules

Originally drm_sched_job_init was the point of no return, after which
drivers really should submit a job. I've split that up, which allows
us to fix this issue pretty easily.

Only thing we have to take care of is to not skip to error paths after
that. Other drivers do this the same for out-fence and similar things.

v2: It's not really a bugfix, just an improvement, since all
drm_sched_job_arm does is reserve the fence number. And gaps should be
fine, as long as the drm_sched_job doesn't escape anywhere at all.

For robustness it's still better to align with other drivers here and
not bail out after job_arm().

v3: I misplaced drm_sched_job_arm by _one_ line! Thanks to Rob for
testing and debug help.

Cc: Rob Clark <robdclark@chromium.org>
Cc: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-arm-msm@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210826093334.1117944-1-daniel.vetter@ffwll.ch