drm/i915/tgl: Gen12 csb support

The CSB format has been reworked for Gen12 to include information on
both the context we're switching away from and the context we're
switching to. After the change, some of the events don't have their
own bit anymore and need to be inferred from other values in the csb.
One of the context IDs (0x7FF) has also been reserved to indicate
the invalid ctx, i.e. engine idle.

Note that the full context ID includes the SW counter as well, but since
we currently only care if the context is valid or not we can ignore that
part.

v2: fix mask size, fix and expand comments (Tvrtko),
    use if-ladder (Chris)

Bspec: 45555, 46144
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-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/20190820102201.29849-1-chris@chris-wilson.co.uk