drm/i915/skl: Make sure to allocate mininum sizes in the DDB

I overlooked the fact that we need to allocate a minimum 8 blocks and
that just allocating the planes depending on how much they need to fetch
from the DDB in proportion of how much memory bw is necessary for the
whole display can lead to cases where we don't respect those minima (and
thus overrun).

So, instead, start by allocating 8 blocks to each active display plane
and then allocate the remaining blocks like before.

v2: Rebase on top of -nightly

Cc: Mahesh Kumar <mahesh1.kumar@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>