drm/amd/display: Create a file dedicated to planes

[Why]
The amdgpu_dm file contains most of the code that works as an interface
between DRM API and DC. As a result, this file becomes very large since
it comprises multiple abstractions such as plane manipulation.

[How]
This commit extracts the plane code to its specific file named
amdgpu_dm_plane. This change does not change anything inside the
functions; the only exception is converting some static functions to a
global function.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>