anv: initialize clear_color_is_zero_one

anv: initialize clear_color_is_zero_one

Fixes following valgrind warning:

   ==12508== Conditional jump or move depends on uninitialised value(s)
   ==12508==    at 0x2CCD8B79: cmd_buffer_begin_subpass (genX_cmd_buffer.c:4599)
   ==12508==    by 0x2CCDA72B: gen9_CmdBeginRenderPass (genX_cmd_buffer.c:5275)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3487>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3487>