drm/amd/display: remove unused func declaration from resource headers

The function resource_validate_ctx_update_pointer_after_copy() is
declared in resource.h but never defined, therefore, remove its
declaration from headers.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>