drm/amd/display: Only flush delta from last command execution

[Why]
We're currently flushing commands that had been previously been
flushed or are currently being processed by the DMCUB when we don't
immediately wait for idle after command execution.

[How]
Avoiding reflushing the data by keeping track of the last wptr.

We'll treat this as the actual rptr by creating a copy of the inbox
and modifying the copy's rptr.

Reviewed-by: Eric Yang <[e-mail]>
Acked-by: Wayne Lin <[e-mail]>
Signed-off-by: Nicholas Kazlauskas <[e-mail]>
Tested-by: Daniel Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/dmub/dmub_srv.h

···
 	struct dmub_srv_base_funcs funcs;
 	struct dmub_srv_hw_funcs hw_funcs;
 	struct dmub_rb inbox1_rb;
+	uint32_t inbox1_last_wptr;
 	/**
 	 * outbox1_rb is accessed without locks (dal & dc)
 	 * and to be used only in dmub_srv_stat_get_notification()

drivers/gpu/drm/amd/display/dmub/src/dmub_srv.c

···
 
 enum dmub_status dmub_srv_cmd_execute(struct dmub_srv *dmub)
 {
+	struct dmub_rb flush_rb;
+
 	if (!dmub->hw_init)
 		return DMUB_STATUS_INVALID;
 
···
 	 * been flushed to framebuffer memory. Otherwise DMCUB might
 	 * read back stale, fully invalid or partially invalid data.
 	 */
-	dmub_rb_flush_pending(&dmub->inbox1_rb);
+	flush_rb = dmub->inbox1_rb;
+	flush_rb.rptr = dmub->inbox1_last_wptr;
+	dmub_rb_flush_pending(&flush_rb);
 
 		dmub->hw_funcs.set_inbox1_wptr(dmub, dmub->inbox1_rb.wrpt);
+
+	dmub->inbox1_last_wptr = dmub->inbox1_rb.wrpt;
+
 	return DMUB_STATUS_OK;
 }