drm/amdgpu: resize VRAM BAR for CPU access on gfx10

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>