drm/amd/display: Fix pointer dereferenced before checking

The value actual_pix_clk_100Hz is dereferencing pointer pix_clk_params
before pix_clk_params is being null checked. Fix this by assigning
pix_clk_params->requested_pix_clk_100hz to actual_pix_clk_100Hz only if
pix_clk_params is not NULL, otherwise just NULL.
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce/dce_clock_source.c

@@ -1254,7 +1254,7 @@ static uint32_t dcn3_get_pix_clk_dividers(
 		struct pixel_clk_params *pix_clk_params,
 		struct pll_settings *pll_settings)
 {
-	unsigned long long actual_pix_clk_100Hz = pix_clk_params->requested_pix_clk_100hz;
+	unsigned long long actual_pix_clk_100Hz = pix_clk_params ? pix_clk_params->requested_pix_clk_100hz : 0;
 	struct dce110_clk_src *clk_src;
 
 	clk_src = TO_DCE110_CLK_SRC(cs);