v3d: Track write reference to the separate stencil buffer.

v3d: Track write reference to the separate stencil buffer.

Otherwise, a blit from separate stencil may fail to flush the job that
initialized it, or new drawing could fail to flush a blit reading from
stencil.

Fixes:
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth32f_stencil8_basic
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth32f_stencil8_scale
dEQP-GLES3.functional.fbo.blit.depth_stencil.depth32f_stencil8_stencil_only
dEQP-GLES3.functional.fbo.msaa.2_samples.depth32f_stencil8
dEQP-GLES3.functional.fbo.msaa.4_samples.depth32f_stencil8