zink: mark fbfetch push sets as non-cached

zink: mark fbfetch push sets as non-cached

these can't be cached, so ensure the value isn't uninitialized

Test case 'KHR-GL46.blend_equation_advanced.blend_all.GL_HARDLIGHT_KHR_all_qualifier'..
==1193311== Conditional jump or move depends on uninitialised value(s)
==1193311==    at 0x634EF05: update_push_ubo_descriptors (zink_descriptors.c:1230)
==1193311==    by 0x634FCC5: zink_descriptors_update (zink_descriptors.c:1412)
==1193311==    by 0x63A5EA1: void zink_draw<(zink_multidraw)1, (zink_dynamic_state)3, true, false>(pipe_context*, pipe_draw_info const*, unsigned int, pipe_draw_indirect_info const*, pipe_draw_start_count_bias const*, unsigned int, pipe_vertex_state*, unsigned int) (zink_draw.cpp:788)
==1193311==    by 0x635A0AF: void zink_draw_vbo<(zink_multidraw)1, (zink_dynamic_state)3, true>(pipe_context*, pipe_draw_info const*, unsigned int, pipe_draw_indirect_info const*, pipe_draw_start_count_bias const*, unsigned int) (zink_draw.cpp:907)
==1193311==    by 0x6174397: tc_call_draw_single (u_threaded_context.c:3150)
==1193311==    by 0x616C403: tc_batch_execute (u_threaded_context.c:211)
==1193311==    by 0x616CA44: _tc_sync (u_threaded_context.c:362)
==1193311==    by 0x61725E9: tc_texture_map (u_threaded_context.c:2274)
==1193311==    by 0x5A9DAE9: pipe_texture_map_3d (u_inlines.h:572)
==1193311==    by 0x5A9EB80: st_ReadPixels (st_cb_readpixels.c:530)
==1193311==    by 0x5A0647A: read_pixels (readpix.c:1178)
==1193311==    by 0x5A0647A: _mesa_ReadnPixelsARB (readpix.c:1195)
==1193311==    by 0x5A06517: _mesa_ReadPixels (readpix.c:1210)

cc: mesa-stable

Acked-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15226>