Disable Gen8 driver code when -DGEN8=OFF

Currently Gen8 is the only Gen that can be fully disabled
(thanks to PR 1302).

Allowing the driver code to be disabled for Gen8 further reduces the
binary size by 277 KiB, or ~11% of the Gen8 media kernels:

    text     data   bss       dec      hex  filename
30821215  1990352  9600  32821167  1f4cfaf  iHD_drv_video.so # -DGEN8=ON
28224191  1990384  9632  30224207  1cd2f4f  iHD_drv_video.so # -DGEN8=OFF before this patch
27950563  1980440  9600  29940603  1c8db7b  iHD_drv_video.so # -DGEN8=OFF after this patch

media_driver/agnostic/media_srcs.cmake

@@ -336,11 +336,11 @@ endif()
 
 media_include_subdirectory(common)
 
-if(ENABLE_REQUIRED_GEN_CODE OR GEN8)
+if(GEN8)
     media_include_subdirectory(gen8)
 endif()
 
-if(ENABLE_REQUIRED_GEN_CODE OR GEN8_BDW)
+if(GEN8_BDW)
     media_include_subdirectory(gen8_bdw)
 endif()
 

media_driver/cmake/linux/media_gen_flags_linux.cmake

@@ -21,7 +21,7 @@
 # TODO: Remove this option when all Gen options can successfully disable
 #       driver code.
 # Status:
-#   GEN8:  TODO
+#   GEN8:  Done
 #   GEN9:  TODO
 #   GEN10: TODO (Remove)
 #   GEN11: TODO

media_driver/linux/media_srcs.cmake

@@ -20,7 +20,7 @@
 
 media_include_subdirectory(common)
 
-if(ENABLE_REQUIRED_GEN_CODE OR GEN8)
+if(GEN8)
     media_include_subdirectory(gen8)
 endif()
 

media_driver/media_interface/media_srcs.cmake

@@ -19,7 +19,7 @@
 # OTHER DEALINGS IN THE SOFTWARE.
 
 
-if(ENABLE_REQUIRED_GEN_CODE OR GEN8_BDW)
+if(GEN8_BDW)
     media_include_subdirectory(media_interfaces_m8_bdw)
 endif()