egl_dri2: fix double free on drm platforms

egl_dri2: fix double free on drm platforms

Earlier commit failed to attribure that for drm platforms one does not
call dri2_create_screen, thus it does not create the screen and
driver_configs but inherits them from the "display" - gbm.

As such wrap cleanup in Platform != _EGL_PLATFORM_DRM to prevent
the issue and still cleanup correctly for non-drm platforms.

v2:
 - Drop the ifdef HAVE_DRM_PLATFORM, reindent the code and fix the
comment block. Suggested by Ken.

Reported-by: Kenneth Graunke <kenneth@whitecape.org>
Reported-by: Mark Janes <mark.a.janes@intel.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-and-tested-by: Kenneth Graunke <kenneth@whitecape.org> (v1)