drm/amd/display: Fix Coverity INTEGER_OVERFLOW within decide_fallback_link_setting_max_bw_policy

[Why]
For addtion (uint8_t) variable + constant 1,
coverity generates message below:
Truncation due to cast operation on "cur_idx + 1" from
32 to 8 bits.

Then Coverity assume result is 32 bits value be saved into
8 bits variable. When result is used as index to access
array, Coverity suspects index invalid.

[How]
Change varaible type to uint32_t.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>