drm/amdgpu: fixup bad vram size on gmc v8

Some boards(like RX550) seem to have garbage in the upper
16 bits of the vram size register.  Check for
this and clamp the size properly.  Fixes
boards reporting bogus amounts of vram.

after add this patch,the maximum GPU VRAM size is 64GB,
otherwise only 64GB vram size will be used.

Signed-off-by: Zongmin <Zhou&lt;zhouzongmin@kylinos.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>