drm/i915: Account for CHV/BXT DPLL clock limitations

CHV/BXT DPLL can't generate frequencies in the 216-240 MHz range.
Account for that when checking whether the HDMI port clock is valid.
This is particularly important for BXT since it can otherwise do
12bpc, and standard 1920x1080p60 CEA modes land right in the middle
of that range when the clock gets multiplied to account for 12bpc.

With the extra checks we will now filter out any mode where both
8bpc and 12bpc clock are within the gap. During modeset we then
pick whichever mode works, favoring 12bpc if both are possible.

12bpc isn't supported on CHV so we simply end up filtering out any
mode where the 8bpc port clock is in the gap.

v2: Fix crtc_clock vs. port_clock fumble in compute_config() (Imre)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-and-tested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>