drm/amd/display: fix mapping to non-allocated address

[Why]
There is an issue mapping non-allocated location of memory.
It would allocate gpio registers from an array out of bounds.

[How]
Patch correct numbers of bounds for using.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Brandon Syu <Brandon.Syu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>