etnaviv: fix stencil operation without Z write

etnaviv: fix stencil operation without Z write

If stencil operations are enabled, we either need the full early or
late ZS stage, even if Z writes are disabled. Empirically the early
stage properly supports stencil operations, just need to be careful to
not mix early tests with late writes when stencil operations modify
the depth buffer.

Fixes piglit tests:
spec@!opengl 1.1@depthstencil-default_fb-copypixels
spec@!opengl 1.1@depthstencil-default_fb-readpixels
spec@!opengl 1.1@depthstencil-default_fb-blit

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19958>