[igt-dev] [PATCH i-g-t] tests/kms_color_chamelium: Fix port_idx

[Joshi, Kunal1]

Hello Bhanu,

Function only returns value in the set [-1, 0, 1, 2, ......, port_count]

> Must be (port_idx >= 0) ? b'coz this condition will pass in case of -2 ...
> - Bhanu