drm/amdgpu: reserve backup pages for bad page retirment

To ensure user has a constant of VRAM accessible in run-time, driver
reserves limit backup pages when init, and return ones when bad pages
retired, to keep no change of unused memory size.

v2: refine codes to calculate badpags threshold

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>