drm/amd/display: update NV12/NV21 as the YUV format

This patch updates the YUV format supported to
NV12 and NV21 and hence updates the offsets.

BUG: SWDEV-119421
TEST: (On Chromium OS for Stoney Only)
    * Executed below tests to see YUV(underlay) & RGB planes on eDP
    plane_test --format XR24 --size 500x100 -p --format NV12 --size
    500x500

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>