drm/i915: Ensure no conflicts with BIOS when updating Dbuf

TGL BIOS seems to enable both DBuf slices ocasionally, depending
how many displays are connected, while i915 according to BSpec
was powering on S1 DBuf slice, until a modeset was done.

This was causing a brief flash during the boot as we were
disabling slice, previously used by BIOS with that.

To prevent this, now we are ensuring tht we are enabling
_at least_ one slice, but if there are more, let's not
power them off.

Fixes: ff2cd863

 ("drm/i915: Correctly map DBUF slices to pipes")
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200213140412.32697-2-stanislav.lisovskiy@intel.com