drm/amd/display: fix dp_hdmi_max_pixel_clk units

[Why]
We are incorrectly using dp_hdmi_max_pixel_clk because the units are not clear.

[How]
Rename to dp_hdmi_max_pixel_clk_in_khz, and change mode timing validation to use
the value correctly.

Signed-off-by: SivapiriyanKumarasamy <sivapiriyan.kumarasamy@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>