drm/amd/display: Add prefix for plane functions

This commit adds the amdgpu_dm_plane_ prefix for all functions in the
amdgpu_dm_plane.c. This change enables an easy way to filter code paths
via ftrace.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>