winsys/drm: Fix out of scope variable usage

In this particular instance, struct member were used outside of the
block where it was defined. Fix this by moving the definition outside of
block.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Fixes: 569f8389 ("winsys/svga: Add support for new surface ioctl, multisample pattern")
Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit 828e1b0b)