[why]
we found using an active DP to HDMI panel that we weren't validating
dp_pixel_format and hardware timing v_front_porch, causing screen to
blank and/or corrupt while attempting a seamless boot.

[how]
added checks during dc_validate_seamless_boot_timing for these values
Signed-off-by: Martin Leung <martin.leung@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>