glsl: Merge precision qualifiers too

glsl: Merge precision qualifiers too

We never noticed this before because we previously didn't enfoce GLSL ES
fragement shader requirements that precision be defined.  There may also
have been some interaction here with the addition of
GL_ARB_shading_language_420pack, but it doesn't appear to me that it
added any new bugs (just perhaps uncovered some old ones).

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: "9.2" <mesa-stable@lists.freedesktop.org>