drm/amd/display: Fix bounds checking in dml2_0 clock table array

drm/amd/display: Fix bounds checking in dml2_0 clock table array

[Why]
Multiple locations in dml2_0 used num_clk_values-1 as array index
without checking if num_clk_values > 0. When num_clk_values is 0,
this results in accessing array index -1, which wraps to 255 for
unsigned types, causing out-of-bounds memory access and potential
crashes.

[How]
Add proper bounds checking using ternary operators to guard all
num_clk_values-1 array accesses. When num_clk_values is 0, return 0
as fallback value instead of accessing invalid memory. This prevents
buffer overflows while maintaining backward compatibility and provides
sensible default behavior for empty clock tables.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Signed-off-by: Gabe Teeger <gabe.teeger@amd.com>
Signed-off-by: Chuanyu Tseng <chuanyu.tseng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>