amd/display: set backlight only if required

[Why]
comparing pwm bl values (coverted) with user brightness(converted)
levels in commit_tail leads to continuous setting of backlight via dmub
as they don't to match.
This leads overdrive in queuing of commands to DMCU that sometimes lead
to depending on load on DMCU fw:

"[drm:dc_dmub_srv_wait_idle] *ERROR* Error waiting for DMUB idle: status=3"

[How]
Store last successfully set backlight value and compare with it instead
of pwm reads which is not what we should compare with.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -540,6 +540,12 @@ struct amdgpu_display_manager {
 	 * cached backlight values.
 	 */
 	u32 brightness[AMDGPU_DM_MAX_NUM_EDP];
+	/**
+	 * @actual_brightness:
+	 *
+	 * last successfully applied backlight values.
+	 */
+	u32 actual_brightness[AMDGPU_DM_MAX_NUM_EDP];
 };
 
 enum dsc_clock_force_state {