drm: add extern C guard for the UAPI headers

CRs-Fixed: 2038080
Change-Id: Id6f4ac3d41fec4804b1449f303d4872bf7a1fd51
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Git-commit: ebbb0e5cfd2ceb1150b1af7f9fcf7aeebfb1b69f
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ebbb0e5cf


Signed-off-by: Manoj Kumar AVM <manojavm@codeaurora.org>