drm/i915: Update execlists context descriptor format commentary

The comments describing the Context Descriptor Format are off by a bit
for the size of the context ID.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461833819-3991-16-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -307,8 +307,9 @@ logical_ring_init_platform_invariants(struct intel_engine_cs *engine)
  * This is what a descriptor looks like, from LSB to MSB:
  *    bits  0-11:    flags, GEN8_CTX_* (cached in ctx_desc_template)
  *    bits 12-31:    LRCA, GTT address of (the HWSP of) this context
- *    bits 32-51:    ctx ID, a globally unique tag (the LRCA again!)
- *    bits 52-63:    reserved, may encode the engine ID (for GuC)
+ *    bits 32-52:    ctx ID, a globally unique tag (the LRCA again!)
+ *    bits 53-54:    mbz, reserved for use by hardware
+ *    bits 55-63:    group ID, currently unused and set to 0
  */
 static void
 intel_lr_context_descriptor_update(struct intel_context *ctx,
@@ -321,7 +322,7 @@ intel_lr_context_descriptor_update(struct intel_context *ctx,
 
 	desc = engine->ctx_desc_template;		   /* bits  0-11 */
 	desc |= lrca;					   /* bits 12-31 */
-	desc |= (lrca >> PAGE_SHIFT) << GEN8_CTX_ID_SHIFT; /* bits 32-51 */
+	desc |= (lrca >> PAGE_SHIFT) << GEN8_CTX_ID_SHIFT; /* bits 32-52 */
 
 	ctx->engine[engine->id].lrc_desc = desc;
 }