drm/i915: Support PCH no display

GEN supports a fusing option which subtracts the PCH display (making the
CPU display also useless). In this configuration MMIO which gets decoded
to a certain range will hang the CPU.

For us, this is sort of the equivalent of having no pipes, and we can
easily modify some code to not do certain things with no pipes.

v2: Moved the num pipes check up in the call chain, and removed extra
checks noted by Daniel. For more details, see:
http://lists.freedesktop.org/archives/intel-gfx/2013-March/025746.html



v3: Drop the intel_setup_overlay check (Daniel)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>