drm/radeon: fixup bad vram size on SI

commit 0ca223b029a261e82fb2f50c52eb85d510f4260e upstream.

Some boards 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.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>