drm/amd/display: Fix incorrect size calculation for loop in dcn401

drm/amd/display: Fix incorrect size calculation for loop in dcn401

[WHY]
fe_clk_en and be_clk_sel have size of 4 but sizeof(fe_clk_en) has
byte size 16 which is lager than the array size.

[HOW]
Use ARRAY_SIZE for calculating size.

This fixes 2 OVERRUN issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>