xf86drm: fix return type for drmIsMaster()

Xserver has struct members named `bool`, which means the last commit
breaks its build with errors like this:

  error: two or more data types in declaration specifiers
  Bool bool;
       ^

Fix this by making it return a 0/1 integer, with the same semantic as
the boolean it was before.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=109587


Fixes: 17dfe3ac "xf86drm: Add drmIsMaster()"
Cc: Christopher James Halse Rogers <christopher.halse.rogers@canonical.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>