drm/i915: add hw state readout/checking for pipe_config

We need to be able to read out the hw state code for a bunch
of reasons:
- Correctly disabling boot-up/resume state.
- Pure paranoia.

Since not all of the pipe configuration is e.g. relevant for
fastboot (or at least we can allow some wiggle room in some
parameters, like the clocks), we need to add a strict_checking
parameter to intel_pipe_config_compare for fastboot.

For now intel_pipe_config_compare should be fully paranoid and
check everything that the hw state readout code supports. Which
for this infrastructure code is nothing.

I've gone a bit overboard with adding 3 get_pipe_config functions:
The ilk version will differ with the next patch, so it's not too
onerous.

v2: Don't check the hw config if the pipe is off, since an enabled,
but dpms off crtc will obviously have tons of difference with the hw
state.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>