drm/amd/display: Tidy up dce120_clock_source_create()

Also change sizeof to be automatic based on type declaration.
Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce120/dce120_resource.c

@@ -398,7 +398,7 @@ struct clock_source *dce120_clock_source_create(
 	bool dp_clk_src)
 {
 	struct dce110_clk_src *clk_src =
-		dm_alloc(sizeof(struct dce110_clk_src));
+		dm_alloc(sizeof(*clk_src));
 
 	if (!clk_src)
 		return NULL;