drm/v3d: decouple adding job dependencies steps from job init

Prep work to enable a job to wait for more than one syncobj before
start. Also get rid of old checkpatch warnings in the v3d_gem file.
No functional changes.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/825f5fdd540b0aa2eb57bd5ff93c0777808b108c.1633016479.git.mwen@igalia.com