drm/radeon: remove redundant assignment to reg

The pointer reg is being assigned a value that is not read, the
exit path via label 'out' never accesses it. The assignment is
redundant and can be removed.

Cleans up clang scan build warning:
drivers/gpu/drm/radeon/radeon_object.c:570:3: warning: Value
stored to 'reg' is never read [deadcode.DeadStores]

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>