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

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 <[email protected]>
Reviewed-by: Harry Wentland <[email protected]>
Acked-by: Alan Liu <[email protected]>
Signed-off-by: Rodrigo Siqueira <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>