drm/i915: clean up media reset on gm45

drm/i915: clean up media reset on gm45

Originally I've thought that this fixes up the reset issues on my
gm45, but that was just a red herring due to b0rked testing.

Still I much prefer writing the right values (all other fields are
reserved) instead of potentially dragging gunk around. Hence also
clear the register to 0 after a reset.

Note that Cspec is a bit confused and doesn't explicitly say that all
the other bits in this register are "reserved, mbz" like usually.
Instead they're marked as "r/o, default value = 0" which semantically
amounts to the same thing.

v2: Stop claiming this fixes anything and return 0 if successful
instead of stack garbage.

v3: Pimp the commit message to explain exactly why I think the docs
allow us to ditch the rmw cycle, spurred by a discussion with Chris.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>