i965/gen8+: Enable GL_OES_viewport_array

i965/gen8+: Enable GL_OES_viewport_array

This patch causes 2 regressions in khronos' gles cts tests
on various intel platforms.
Failing tests:
ES3-CTS.functional.state_query.integers.viewport_getinteger
ES3-CTS.functional.state_query.integers.viewport_getfloat

Here is an explanation of what's causing the failures:

CTS tests are not clamping the x, y location of the viewport's
bottom-left corner as recommended by ARB_viewport_array and
OES_viewport_array:
"The location of the viewport's bottom-left corner, given by (x,y), are
 clamped to be within the implementation-dependent viewport bounds range.
 The viewport bounds range [min, max] tuple may be determined by
 calling GetFloatv with the symbolic constant VIEWPORT_BOUNDS_RANGE_OES"

Khronos CTS merge request to fix the test case:
https://gitlab.khronos.org/opengl/cts/merge_requests/399

V2: Initialize the relevant variables for GL_OES_viewport_array on gen8+

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>