accel: Build sub-directories based on config options

When accel drivers are disabled do not process into
sub-directories and create built-in archives:

  AR      drivers/accel/habanalabs/built-in.a
  AR      drivers/accel/ivpu/built-in.a

Fixes: 35b13763 ("accel/ivpu: Introduce a new DRM driver for Intel VPU")
Signed-off-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: Jacek Lawrynowicz <jacek.lawrynowicz@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301162508.3963484-1-stanislaw.gruszka@linux.intel.com

drivers/accel/Makefile

 # SPDX-License-Identifier: GPL-2.0-only
 
-obj-y	+= habanalabs/
-obj-y	+= ivpu/
+obj-$(CONFIG_DRM_ACCEL_HABANALABS)	+= habanalabs/
+obj-$(CONFIG_DRM_ACCEL_IVPU)		+= ivpu/