drm/i915: Do not enable DRRS when PSR is enabled

Some platforms do not support PSR and DRRS simultaneously.
Visual artifacts and flickering were reported on BDW HP Spectre
x360 Convertible. Deferring to PSR when both PSR and DRRS are
supported by the panel.

V2: Minor code-style changes suggested by Rodrigo
V3: Add a WARN_ON during PSR init suggested by Dhinakaran
    Correct debug message,title suggested by Jani

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101111
Cc: Nicholas Stommel <nicholas.stommel@gmail.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Clinton Taylor <clinton.a.taylor@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170914181641.24393-1-radhakrishna.sripada@intel.com