i965: Add a driconf option to force GLSL extension behavior to "warn".

i965: Add a driconf option to force GLSL extension behavior to "warn".

This can be used to work around broken application behavior, like in
Unigine where it attempts to use texture arrays without declaring
either "#extension GL_EXT_texture_array : enable" or "#version 130".

NOTE: This is a candidate for the 8.0 branch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>