drm/i915: Calculate edram size

With gen9+ the edram capabilities are defined so
that we can calculate the edram (ellc) size accordingly.

Note that there are undefined combinations for some subset of
edram capability bits. Return the closest size for undefined indexes.
Even if we get it wrong with beginning of future gen enabling, the size
information is currently only used for boot message and in debugfs entry.

v2: Use function instead of hard to read macro (Daniel)
v3: s/INTEL_INFO/INTEL_GEN (Matthew)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460557604-7126-2-git-send-email-mika.kuoppala@intel.com