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>