mesa: Decode GL_CLIP_DISTANCE[67] properly in _mesa_set_enable().

mesa: Decode GL_CLIP_DISTANCE[67] properly in _mesa_set_enable().

In order to support 8 clip distances, we need to properly decode when
the user sets the GL_CLIP_DISTANCE6 and GL_CLIP_DISTANCE7 enable
flags.

For clarity, this patch changes the names GL_CLIP_PLANE[0-5] in the
switch statement to the equivalent names GL_CLIP_DISTANCE[0-5], since
the GL_CLIP_PLANE names are deprecated.

Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Brian Paul <brianp@vmware.com>