package/gstreamer1/gst1-plugins-base: fix dispmanx option

There is a typo in the handling of the
BR2_PACKAGE_GST1_PLUGINS_BASE_LIB_OPENGL_DISPMANX option: we're adding
dispmax to GST1_PLUGINS_BASE_WINSYS_LIST, which causes the following
build failure:

meson.build:1:0: ERROR: Options "dispmax" are not in allowed choices: "x11, wayland, win32, cocoa, dispmanx, viv-fb, gbm, auto"

We fix this by using the proper option name, "dispmanx" instead of the
slightly incorrect "dispmax".

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>