amd-gpu: Don't undefine READ and WRITE

Remove the undefinition of READ and WRITE because these constants may be
used elsewhere in subsequently included header files, thus breaking them.

These constants don't actually appear to be used in the driver, so the
undefinition seems pointless.

Fixes: 4562236b

 ("drm/amd/dc: Add dc display driver (v2)")
Signed-off-by: David Howells <dhowells@redhat.com>