mesa: Move {split,merge}_location_offset to uniforms.h

mesa: Move {split,merge}_location_offset to uniforms.h

Prepend _mesa_uniform_ to the names and rework the calling
convention.  The calling convention was changed for a couple reasons.

1. Having a single variable named 'location' have completely different
meanings at different places in the function is confusing.  Before
calling split_location_offset the location is the encoded value
returned by glGetUniformLocation.  After calling split_location_offset
it's the index of the uniform in the gl_uniform_list::Uniforms array.

2. In a later commit the original value of 'location' is needed after
split_location_offset has been called.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Tested-by: Tom Stellard <thomas.stellard@amd.com>