i965: Advertise 8 bits subpixel precision for viewport bounds on gen6+
We use floating-points for viewport bounds so VIEWPORT_SUBPIXEL_BITS should reflect this. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105975 Signed-off-by:Danylo Piliaiev <danylo.piliaiev@globallogic.com> Reviewed-by:
Lionel Landwerlin <lionel.g.landwerlin@intel.com>