glsl: Fix write mask in matrix-from-matrix constructors.

glsl: Fix write mask in matrix-from-matrix constructors.

If the matrix being constructed was larger than the source matrix, it
would overwrite the lower-right part of the matrix with the wrong
values, rather than leaving it as the identity matrix.

For example, constructing a mat4 from a mat2 should only use a writemask
of "xy" when copying from the source, but was using "xyzw".

Fixes the code generated by piglit test constructor-23.vert.