softpipe: zero-out entire clear_flags array in sp_tile_cache_flush_clear()

softpipe: zero-out entire clear_flags array in sp_tile_cache_flush_clear()

Before, we only cleared the flags for the active tiles (the ones inside
the framebuffer bound).  The problem is if we later bound a different,
larger surface to the tile cache we'd have some stale clear-flags still
set (and mistakenly clear some tiles in the new surface).

Fixes fd.o bug 26932.