drm/atomic: Filter out redundant DPMS calls

Video players (eg. mpv) do periodic XResetScreenSaver() calls to
keep the screen on while the video playing. The modesetting ddx
plumbs these straight through into the kernel as DPMS setproperty
ioctls, without any filtering whatsoever. When implemented via
atomic these end up as empty commits on the crtc (which will
nonetheless take one full frame), which leads to a dropped
frame every time XResetScreenSaver() is called.

Let's just filter out redundant DPMS property changes in the
kernel to avoid this issue.

v2: Explain the resulting commits a bit better (Sima)
    Document the behaviour in uapi docs (Sima)

Cc: stable@vger.kernel.org
Testcase: igt/kms_flip/flip-vs-dpms-on-nop
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250219160239.17502-1-ville.syrjala@linux.intel.com

drivers/gpu/drm/drm_atomic_uapi.c

@@ -956,6 +956,10 @@ int drm_atomic_connector_commit_dpms(struct drm_atomic_state *state,
 
 	if (mode != DRM_MODE_DPMS_ON)
 		mode = DRM_MODE_DPMS_OFF;
+
+	if (connector->dpms == mode)
+		goto out;
+
 	connector->dpms = mode;
 
 	crtc = connector->state->crtc;

drivers/gpu/drm/drm_connector.c

@@ -1427,6 +1427,10 @@ EXPORT_SYMBOL(drm_hdmi_connector_get_output_format_name);
  * 	callback. For atomic drivers the remapping to the "ACTIVE" property is
  * 	implemented in the DRM core.
  *
+ * 	On atomic drivers any DPMS setproperty ioctl where the value does not
+ * 	change is completely skipped, otherwise a full atomic commit will occur.
+ * 	On legacy drivers the exact behavior is driver specific.
+ *
  * 	Note that this property cannot be set through the MODE_ATOMIC ioctl,
  * 	userspace must use "ACTIVE" on the CRTC instead.
  *