drm/radeon: Fix ENOSYS with better fitting error codes in radeon_gem.c

Replace the error code from 'ENOSYS' to 'EOPNOTSUPP' for unimplemented
radeon_gem_pread_ioctl & radeon_gem_pwrite_ioctl

Fixes the following:

WARNING: ENOSYS means 'invalid syscall nr' and nothing else.

Cc: Christian König <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Signed-off-by: Srinivasan Shanmugam <[e-mail]>
Acked-by: Alex Deucher <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/radeon/radeon_gem.c

···
 {
 	/* TODO: implement */
 	DRM_ERROR("unimplemented %s\n", __func__);
-	return -ENOSYS;
+	return -EOPNOTSUPP;
 }
 
 int radeon_gem_pwrite_ioctl(struct drm_device *dev, void *data,
···
 {
 	/* TODO: implement */
 	DRM_ERROR("unimplemented %s\n", __func__);
-	return -ENOSYS;
+	return -EOPNOTSUPP;
 }
 
 int radeon_gem_create_ioctl(struct drm_device *dev, void *data,