drm/radeon: Use direct mapping for fast fb access on RS690

This patch allows the CPU to map the stolen vram segment
directly rather than going through the PCI BAR.  This
significantly improves performance for certain workloads with
a properly patched ddx.

Use radeon.fastfb=1 to enable it (disabled by default).
Currently only supported on RS690, but support for RS780/880
and newer APUs may be added eventually.

Signed-off-by: Samuel Li <samuel.li@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>