libdrm: Make some drm headers compatible with gcc -std=c89 -pedantic

The following minor changes were needed to these headers:
 * Convert // comments to /* */
 * No , after final member of enum

With these changes, these header files can be included by a program that
is built with gcc options:
  -std=c89 -Werror -pedantic

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>