drm/vmwgfx: Use kasprintf

Use kasprintf instead of combination of kmalloc and sprintf. Also,
remove the local variables used for storing the string length as they
are not required now.

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>