We apparently don't have anything else making sure that it's flushed in
between use as a render target and use as a texture source, so bypass-mode
depth texture sampling could get stale data.

Fixes consistent (as far as I could see) failures in FD_MESA_DEBUG=nogmem
on:

dEQP-GLES31.functional.texture.multisample.samples_*.use_texture_depth_2d
dEQP-GLES31.functional.stencil_texturing.render.depth24_stencil8_draw

Cc: mesa-stable
Part-of: <mesa/mesa!8146>