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>
(cherry picked from commit 25ec806e)