i965/fs: move the fs_reg::smear() from get_timestamp() to the callers

i965/fs: move the fs_reg::smear() from get_timestamp() to the callers

We're about to reuse get_timestamp() for the nir_intrinsic_shader_clock.
In the latter the generalisation does not apply, so move the smear()
where needed. This also makes the function analogous to the vec4 one.

v2: Tweak the comment - The caller -> We (Matt, Connor).
v3: More comment tweaks (Connor)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>