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

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 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.

Reviewed-by: Eric Anholt <eric@anholt.net>