drm/radeon/radeon_vm: Fix function naming disparities

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/radeon/radeon_vm.c:61: warning: expecting prototype for radeon_vm_num_pde(). Prototype was for radeon_vm_num_pdes() instead
 drivers/gpu/drm/radeon/radeon_vm.c:642: warning: expecting prototype for radeon_vm_update_pdes(). Prototype was for radeon_vm_update_page_directory() instead

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>