drm/v3d: Store a pointer to `struct v3d_file_priv` inside each job

drm/v3d: Store a pointer to `struct v3d_file_priv` inside each job

Instead of storing a pointer to the DRM file data, store a pointer
directly to the private V3D file struct. No functional change, this
commit only avoids multiple levels of pointer indirection and makes
the code more straightforward.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Link: https://lore.kernel.org/r/20250826-v3d-queue-lock-v3-1-979efc43e490@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>