i965: fix mask used to write to clip distance registers when gen>6

i965: fix mask used to write to clip distance registers when gen>6

In gen6 and above, clip distances 0-3 are written to message register
3's xyzw components, and 4-7 to message register 4's xyzw components.
Therefore when when writing the clip distances we need to examine the
lower 2 bits of the clip distance index to see which component to
write to.

emit_vertex_write() was examining the lower 3 bits, causing clip
distances 4-7 not to be written correctly.

Fixes piglit test vs-clip-vertex-01.shader_test