i915g: fixup clear params emission

i915g: fixup clear params emission

Docs say that default shader input color input need to be spec
as ARGB8888. And a clear rect prim essentially uses this value
instead of default diffuse. Depth on the other hands is an ieee
32 bit float. Clear stencil is U8.

Completely different are the clear values for zone init prims.
These are speced in the actual output pixel layout (and need
to be repeated for 16 bit formats).

Clear up the confusion by adding some comments.

v2: Retain the target swizzling support added by Stephan Marchesin.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>