drm/radeon: Clean up assignment of TTM placement lpfn member for pinning

This sets the lpfn member to 0 instead of the full domain size. TTM uses
the full domain size when lpfn is 0.

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon_object.c
Simple merge