drm/i915/skl: Add debug messages at the start/end of DMC firmware loading

It's handy to have debug message for the "big" events and this one
qualifies IMHO. Also helpful to see what's happening while we're loading
the firwmare and how much time it takes.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>