drm/amd/display: Implement zpos property

Despite certain GPUs supporting multiple overlay planes already in
AMDGPU, the driver did not expose the zpos property which is required
for userspace to take advantage of multiple overlay planes in any
meaningful way.

The driver was already hooked up to normalized_zpos, but was just
missing the exposure of it.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Melissa Wen <mwen@igalia.com>
Cc: Simon Ser <contact@emersion.fr>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Joshua Ashton <joshua@froggi.es>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>