nvfx: reset nvfx->hw_zeta

nvfx: reset nvfx->hw_zeta

If nvfx_framebuffer prepare and validate were called successively with
fb->zsbuf not NULL and then NULL, nvfx->hw_zeta would contain garbage and
this would cause failures in nvfx_framebuffer_relocate/OUT_RELOC(hw_zeta).

This was triggered by piglit/texwrap 2D GL_DEPTH_COMPONENT24 and caused
first a 'write to user buffer!!' error in libdrm and then worse things.

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>