drm/i915: hw state readout support for pipe timings

This does duplicate the logic in intel_crtc_mode_get a bit, but the
issue is that we also should handle interlace modes and other insanity
correctly.

Hence I've opted for a sligthly more elaborate route where we first
read out the crtc timings for the adjusted mode, and then optionally
(not sure if we really need it) compute the modeline from that.

v2: Also read out the pipe source dimensions into the requested mode.

v3: Rebase on top of the moved cpu_transcoder.

v4: Simplify CHECK_FLAGS logic as suggested by Chris Wilson. Also
properly #undef that macro again.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com> (v3)
[danvet: Use the existing mask for interlaced bits, spotted by Mika.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>