drm/amd/display: Check index for aux_rd_interval before using

aux_rd_interval has size of 7 and should be checked.

This fixes 3 OVERRUN and 1 INTEGER_OVERFLOW issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@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>