drm/amd/display: fix array index out of bound error in bios parser

[Why&How]
Firmware headers dictate that gpio_pin array only has a size of 8. The
count returned from vbios however is greater than 8.

Fix this by not using array indexing but incrementing the pointer since
gpio_pin definition in atomfirmware.h is hardcoded to size 8

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>