drm/amd/display: Include missing header

The file amdgpu_dm_plane.c missed the header amdgpu_dm_plane.h, which
resulted on the following warning:

drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:1046:5:
warning: no previous prototype for 'fill_dc_scaling_info'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:1222:6:
warning: no previous prototype for 'handle_cursor_update'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:152:6:
warning: no previous prototype for 'modifier_has_dcc'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:1576:5:
warning: no previous prototype for 'amdgpu_dm_plane_init'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:157:10:
warning: no previous prototype for 'modifier_gfx9_swizzle_mode'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:752:5:
warning: no previous prototype for 'fill_plane_buffer_attributes'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:83:31:
warning: no previous prototype for 'amd_get_format_info'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:88:6:
warning: no previous prototype for 'fill_blending_from_plane_state'
[-Wmissing-prototypes]
drivers/gpu/drm/amd/amdgpu/../display/amdgpu_dm/amdgpu_dm_plane.c:992:5:
warning: no previous prototype for 'dm_plane_helper_check_state'
[-Wmissing-prototypes]

Therefore, include the missing header on the file and turn global functions
that are not used outside of the file into static functions.

Fixes: 5d945cbc

 ("drm/amd/display: Create a file dedicated to planes")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>