ANDROID: drivers: gpu: drm: fix export symbol type

In commit c9b38cbd ("ANDROID: drivers: gpu: drm: export
drm_mode_convert_umode symbol") a new symbol was exported, but it should
have been set as a _GPL symbol.

Fix this up by properly changing the export type.

Bug: 139653858
Cc: Shashank Babu Chinta Venkata <sbchin@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ib33860bcba4676c9d0538f58b500d094b8357867