drm/radeon: program auxch directly (v2)

The atombios tables have an unfortunate restriction on only
being able to write 12 bytes, MST really wants 16-bytes here,
and since the hw can do it, we should just write directly to it.

This uses a module option to allow for it now, and maybe
we should provide the old code as a fallback for a while.

v2: (agd5f)
- move registers to a proper register header
- only enable on DCE5+
- enable by default on DCE5+
- Switch pad to aux mode before using it
- reformat instance handling to better match the
  rest of the driver

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_drv.c

@@ -190,6 +190,7 @@ int radeon_deep_color = 0;
 int radeon_use_pflipirq = 2;
 int radeon_bapm = -1;
 int radeon_backlight = -1;
+int radeon_auxch = -1;
 
 MODULE_PARM_DESC(no_wb, "Disable AGP writeback for scratch registers");
 module_param_named(no_wb, radeon_no_wb, int, 0444);
@@ -275,6 +276,9 @@ module_param_named(bapm, radeon_bapm, int, 0444);
 MODULE_PARM_DESC(backlight, "backlight support (1 = enable, 0 = disable, -1 = auto)");
 module_param_named(backlight, radeon_backlight, int, 0444);
 
+MODULE_PARM_DESC(auxch, "Use native auxch experimental support (1 = enable, 0 = disable, -1 = auto)");
+module_param_named(auxch, radeon_auxch, int, 0444);
+
 static struct pci_device_id pciidlist[] = {
 	radeon_PCI_IDS
 };