drm/i915: gen7_setup_fixed_func_scheduler() actually implements WaVSThreadDispatchOverride

The current comments indicate that this function implements
WaVSRefCountFullforceMissDisable, which is only true for HSW.
The original purpose of the function is to implement
WaVSThreadDispatchOverride (and a bit more). Fix up the comments
to match reality.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>