build: opencv: disable opencv deps if it's not installed

When hotdoc documentation is enabled and opencv plugin is set as auto-detected, but the library isn't installed, meson configuration fails with this message:

../subprojects/gst-plugins-bad/docs/meson.build:139:21: ERROR: Unknown variable "gstopencv_dep"

This patch fixes this case defined gstopencv_dep as disabler() when dependencies aren't found.