egl/dri2: Don't dlclose() the driver on dri2_load_driver_common failure

... because it's wrong to do so. The error path out of dri2_initialize_drm ends with dri2_display_destroy, which calls functions in the vtable we're trying to set up, so if we dlclose the driver then those function pointers will point off into space and things crash.

Noticed this because after !1923 (merged) eglinfo would crash when setting up the GBM platform. This was something of a cascade failure, because my kernel is too old for DRM_IOCTL_I915_GETPARAM to work without DRM_AUTH, so i965 wouldn't load. platform_drm.c then got very confused when it tries to load swrast as a dri2 driver.