drm/i915/wopcm: Check WOPCM layout separately from calculations

We can do WOPCM partitioning using rough estimates and limits
and perform detailed check as separate step.

v2: oops! s/max/min
v3: consolidate overflow checks (Daniele)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190816105501.31020-3-michal.wajdeczko@intel.com