[SKL] Disable display power well

Display power well can cause issues like blank screen when logging in after resume from suspend, so it's best to disable it for now like upstream did in v4.4-rc1:

commit 1b0e3a049efe471c399674fd954500ce97438d30
Author: Imre Deak <email address hidden>
Date: Thu Nov 5 23:04:11 2015 +0200

    drm/i915/skl: disable display side power well support for now

    The display power well support on this platform is in a somewhat broken
    state atm, so disable it by default.

    This in effect will get rid of incorrect assert WARNs about the CSR/DMC
    firmware not being loaded during power well toggling. It also removes a
    problem during driver loading where a register is accessed while its
    backing power well is down, resulting in another WARN. Until we come up
    with the root cause of the second problem and the proper fix for both
    issues, keep all display side power wells on.

    Also clarify a bit the option description.

    Reported-by: Dave Airlie <email address hidden>
    Reference: http://mid<email address hidden>
    Signed-off-by: Imre Deak <email address hidden>
    Reviewed-by: Ville Syrjälä <email address hidden>
    Link: http://patchwork.freedesktop<email address hidden>
    Signed-off-by: Jani Nikula <email address hidden>