drm/amdgpu: fix description of vm_size module parameter (v2)

default is 8GB, not 4GB.

v2: fix fallback setting when the user provides an invalid input

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
Simple merge

diff --cc drivers/gpu/drm/amd/amdgpu/amdgpu_drv.c
Simple merge