kernel: rename symbol in kernel 4.19 config

A symbol was renamed upstream starting from kernel 4.18 [1]

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.0-rc5&id=34b13e5e4641c0e9e0aad471a6d8dfb7999276f1

target/linux/generic/config-4.19

@@ -1218,7 +1218,7 @@ CONFIG_DQL=y
 # CONFIG_DRM_BOCHS is not set
 # CONFIG_DRM_CIRRUS_QEMU is not set
 # CONFIG_DRM_DEBUG_MM is not set
-# CONFIG_DRM_DEBUG_MM_SELFTEST is not set
+# CONFIG_DRM_DEBUG_SELFTEST is not set
 # CONFIG_DRM_DP_AUX_CHARDEV is not set
 # CONFIG_DRM_DP_CEC is not set
 # CONFIG_DRM_DUMB_VGA_DAC is not set