drm/xe: Add build on bug to assert page fault queue works

If PF_QUEUE_NUM_DW % PF_MSG_LEN_DW != 0 then the page fault queue logic
does not work when wrapping occurs. Add a build bug on to assert
PF_QUEUE_NUM_DW % PF_MSG_LEN_DW == 0 to enforce this restriction and
document the code.

v2:
- s/NUM_PF_QUEUE/PF_QUEUE_NUM_DW (Brian)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>

drivers/gpu/drm/xe/xe_gt_pagefault.c

@@ -328,6 +328,11 @@ int xe_guc_pagefault_handler(struct xe_guc *guc, u32 *msg, u32 len)
 	u32 asid;
 	bool full;
 
+	/*
+	 * The below logic doesn't work unless PF_QUEUE_NUM_DW % PF_MSG_LEN_DW == 0
+	 */
+	BUILD_BUG_ON(PF_QUEUE_NUM_DW % PF_MSG_LEN_DW);
+
 	if (unlikely(len != PF_MSG_LEN_DW))
 		return -EPROTO;