drm/mali-dp: Improve writeback handling for DP500.

Mali DP500 operates in continuous writeback mode (writes frame content
until stopped) and it needs special handling in order to behave like
a one-shot writeback engine. The original state machine added for DP500
was a bit fragile, as it did not handle correctly cases where a new
atomic commit was in progress when the SE IRQ happens and it would
commit some partial updates.

Improve the handling by adding a parameter to the set_config_valid()
function to clear the config valid bit in hardware before starting a
new commit and by introducing a MW_RESTART state in the writeback
state machine to cater for the case where a new writeback commit
gets submitted while the last one is still being active.

Reported-by: Brian Starkey <brian.starkey@arm.com>
Reviewed-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>

drivers/gpu/drm/arm/malidp_hw.h

@@ -152,12 +152,13 @@ struct malidp_hw {
 	bool (*in_config_mode)(struct malidp_hw_device *hwdev);
 
 	/*
-	 * Set configuration valid flag for hardware parameters that can
-	 * be changed outside the configuration mode. Hardware will use
-	 * the new settings when config valid is set after the end of the
-	 * current buffer scanout
+	 * Set/clear configuration valid flag for hardware parameters that can
+	 * be changed outside the configuration mode to the given value.
+	 * Hardware will use the new settings when config valid is set,
+	 * after the end of the current buffer scanout, and will ignore
+	 * any new values for those parameters if config valid flag is cleared
 	 */
-	void (*set_config_valid)(struct malidp_hw_device *hwdev);
+	void (*set_config_valid)(struct malidp_hw_device *hwdev, u8 value);
 
 	/*
 	 * Set a new mode in hardware. Requires the hardware to be in