docs: drm/amdgpu: Resolve build warnings

Some of the documentation formatting could be improved
which will resolve some Sphinx amdgpu build warnings e.g

WARNING: Unexpected indentation.
WARNING: Block quote ends without a blank line; unexpected unindent.
WARNING: Inline emphasis start-string without end-string.

Signed-off-by: Adam Zerella <adam.zerella@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>