drm/i915: Add ddb size field to device info structure

Adding the ddb size into the devide info will avoid
platform checks while computing wm.

v2: Added comment and WARN_ON if ddb size is zero.(Jani)
v3: Added WARN_ON at the right place.(Jani)

Suggested-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Deepak M <m.deepak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473931870-7724-1-git-send-email-m.deepak@intel.com