mesa: don't clamp just based on ARB_viewport_array extension

mesa: don't clamp just based on ARB_viewport_array extension

The ARB_viewport_array spec says:

"Dependencies
    OpenGL 1.0 is required.

    OpenGL 3.2 or the EXT_geometry_shader4 or ARB_geometry_shader4 extensions
    are required.

    This extension is written against the OpenGL 3.2 (Compatibility)
    Specification."

As such, we should ignore it for GLES2 contexts.

Fixes:
dEQP-GLES2.functional.state_query.integers.viewport_getinteger
dEQP-GLES2.functional.state_query.integers.viewport_getfloat

on llvmpipe and virgl.

v2: Use _mesa_has_* (Ilia)

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Cc: 17.3 18.0 <mesa-stable@lists.freedesktop.org>