drm/i915: Allocate enough DDB for the cursor

Currently we just assume that 32 or 8 blocks of ddb is sufficient
for the cursor. The 32 might be, but the 8 is certainly not. The
minimum we need is at least what level 0 watermarks need, but that
is a bit restrictive, so instead let's calculate what level 7
would need for a 256x256 cursor. We'll use that to determine the
fixed ddb allocation for the cursor. This way the cursor will never
be responsible for missing out on deeper power saving states.

v2: Loop to make sure this works even if some wm levels are
    totally disabled (latency==0)

Cc: Neel Desai <neel.desai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20190319160311.23529-1-ville.syrjala@linux.intel.com