drm/amd/display: add prefix to amdgpu_dm_plane.h functions

The amdgpu_dm_plane.h functions didn't have names that indicated where
they were declared.

To better filter results in debug tools like ftrace, prefix these
functions with 'amdgpu_dm_plane_'.

Note that we may want to make this same change in other files like
amdgpu_dm_crtc.h.

Signed-off-by: David Tadokoro <davidbtadokoro@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>