drm/amd/display: update dcn315 clock table read

[Why & How]
Make dcn315 base its clock table off dcfclk rather than fclk.

This change also adds some sanity checking to make sure an
empty pmfw table does not result in invalid dal clocks.

Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>