drm/amd/display: Fix Coverity INTEGER_OVERFLOW within dal_gpio_service_create

[Why]
For subtraction, coverity reports integer overflow
warning message when variable type is uint32_t.

[How]
Change variable type to int32_t.

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>