Try to resize BAR0 to let CPU access all of VRAM on Navi. Syncs
code with previous gfx generations from commit d6895ad3
("drm/amdgpu: resize VRAM BAR for CPU access v6").

Signed-off-by: Alan Swanson <reiver@improbability.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>