i965: Fix brw_math1 with scalar argument in gen6 FS.

i965: Fix brw_math1 with scalar argument in gen6 FS.

The docs claim two conflicting things: One, that a scalar source is
supported.  Two, source hstride must be 1 and width must be exec size.
So splat a constant argument out into a full reg to operate on, since
violating the second set of constraints is clearly failing.

The alternative here might be to do a 1-wide exec on a constant
argument for math1.  It would probably save cycles too.  But I'll
leave that for the glsl2-965 branch.

Fixes glsl-algebraic-div-one-2.shader_test.