drm/vmwgfx: Use kmemdup instead of kmalloc and memcpy

When some other buffer is immediately copied into allocated region.
Replace calls to kmalloc followed by a memcpy with a direct
call to kmemdup.

Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Reviewed-by: Sinclair Yeh <syeh@vmare.com>