drm/stm: dsi: replace warning by debug message

To avoid too many warning during low power test, replace warning by
a drm debug message. This is a miss and should not be upstream.

Change-Id: I8600fac164d2bb91abbc58ce7648b35b7da09ba8
Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/441014
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>

Author: Yannick Fertre

drivers/gpu/drm/stm/dw_mipi_dsi-stm.c

@@ -493,7 +493,7 @@ static void dw_mipi_dsi_clk_disable(struct clk_hw *clk)
 			dsi_clear(dsi, DSI_WRPCR, WRPCR_REGEN | WRPCR_BGREN);
 		}
 	} else {
-		DRM_WARN("Warning peripheral clock was not enabled!\n");
+		DRM_DEBUG_DRIVER("Warning peripheral clock was not enabled!\n");
 	}
 }