drm/i915: Make sure cdclk is high enough for DP audio on VLV/CHV

drm/i915: Make sure cdclk is high enough for DP audio on VLV/CHV

On VLV/CHV there is some kind of linkage between the cdclk frequency
and the DP link frequency. The spec says:
"For DP audio configuration, cdclk frequency shall be set to
 meet the following requirements:
 DP Link Frequency(MHz) | Cdclk frequency(MHz)
 270                    | 320 or higher
 162                    | 200 or higher"

I suspect that would more accurately be expressed as
"cdclk >= DP link clock", and in any case we can express it like
that in the code because of the limited set of cdclk (200, 266,
320, 400 MHz) and link frequencies (162 and 270 MHz) we support.

Without this we can end up in a situation where the cdclk
is too low and enabling DP audio will kill the pipe. Happens
eg. with 2560x1440 modes where the 266MHz cdclk is sufficient
to pump the pixels (241.5 MHz dotclock) but is too low for
the DP audio due to the link frequency being 270 MHz.

v2: Spell out the cdclk and link frequencies we actually support

Cc: stable@vger.kernel.org
Tested-by: Stefan Gottwald <gottwald@igel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111149
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717114536.22937-1-ville.syrjala@linux.intel.com
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>