drm/i915: Disable DRRS on IVB/HSW port != A

drm/i915: Disable DRRS on IVB/HSW port != A

[ Upstream commit ee59792c97176f12c1da31f29fc4c2aab187f06e ]

Currently we allow DRRS on IVB PCH ports, but we're missing a
few programming steps meaning it is guaranteed to not work.
And on HSW DRRS is not supported on anything but port A ever
as only transcoder EDP has the M2/N2 registers (though I'm
not sure if HSW ever has eDP on any other port).

Starting from BDW all transcoders have the dynamically
reprogrammable M/N registers so DRRS could work on any
port.

Stop initializing DRRS on ports where it cannot possibly work.

Cc: stable@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128103757.22461-11-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit f0d4ce59f4d48622044933054a0e0cefa91ba15e)
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>