glsl: Extract interpretation of interpolation to its own function.

glsl: Extract interpretation of interpolation to its own function.

In future patches, we will need this in order to interpret
interpolation qualifiers that appear inside interface blocks.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>