drm/i915: Instrument PSR parameter for debuging with link standby x link off.

Unfortunately we don't know all panels and platforms out there and we
found internal prototypes without VBT proper set but where only
link in standby worked well.

So, before enable PSR by default let's instrument the PSR parameter
in a way that we can identify different panels out there that might
require or work better with link standby mode.

It is also useful to say that for backward compatibility I'm not
changing the meaning of this flag. So "0" still means disabled
and "1" means enabled with full support and maximum power savings.

v2: Use positive value instead of negative for different operation mode
    as suggested by Daniel.

v3: As Paulo suggested use 2 to force link standby and 3 to force link
    fully on. Also split the link_standby introduction in a separated patch.

v4: Use DRM_ERROR for link off request on platforms that don't support and
    Remove the quirk promise.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1454356928-19779-1-git-send-email-rodrigo.vivi@intel.com