st/mesa: help fix stencil border color for GL_DEPTH_STENCIL textures

GL_STENCIL_INDEX uses GL_INTENSITY for the border color, which is nicer
to hardware that doesn't read the stencil border value from the X channel.

This fixes a bunch of dEQP tests on Vega & Raven.

Cc: 18.1 18.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 725e8ad5)