drm/amd/display: Check gpio_id before used as array index

drm/amd/display: Check gpio_id before used as array index

[WHY & HOW]
GPIO_ID_UNKNOWN (-1) is not a valid value for array index and therefore
should be checked in advance.

This fixes 5 OVERRUN issues reported by Coverity.

Reviewed-by: Harry Wentland <harry.wentland@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>