glsl: Remove shader stage checking for extension handling.

glsl: Remove shader stage checking for extension handling.

Certain extensions only add functionality to particular shader stages.
(For example, ARB_draw_instanced only adds variables to the vertex
shader stage.)

Previously, we only allowed such extensions to be enabled in the shader
stages where they're useful.  However, I've never found any text which
mandates that behavior; in my opinion, you should be able to turn on
extensions in any shader stage, even if they have no effect.

Fixes Piglit tests glslparsertest/glsl2/draw_buffers-05.vert and
ARB_draw_instanced/preprocessor/feature-macro-enabled.frag.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=29185
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>