i915: Add optional support for ARB_fragment_shader under a driconf option.

i915: Add optional support for ARB_fragment_shader under a driconf option.

Other vendors have enabled ARB_fragment_shader as part of OpenGL 2.0
enablement even on hardware like the 915 with no dynamic branching or
dFdx/dFdy support.  But for now we'll leave it disabled because we don't
do any flattening of ifs or loops, which is rather restrictive.

This support is not complete, and may be unstable depending on your shaders.
It passes 10/15 of the piglit glsl tests, but hangs on glean glsl1.