The pipe and plane _are_ disabled when we call this. So replace it
all with the corresponding assert (as self-documenting code) and
rip out all the lore.

Checking for a disabled plane would require us to export those macros
from intel_display.c, but if the pipe is off the plane isn't working
either. So this single check is good enough.
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>