drm/i915: keep track of used entries in MOCS table

Instead of considering we have defined entries for any index in the
table, let's keep track of the ones we explicitly defined. This will
allow Gen 11 to have it's new table defined in which we have holes of
undefined entries.

Repeated comments about the meaning of undefined entries were removed
since they are overly verbose and copy-pasted in several functions: now
the definition is in the top only.

v2: add helper function to get the index (from Chris)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190124000604.18861-6-lucas.demarchi@intel.com