drm/vmwgfx: Fix a 64bit regression on svga3

Register accesses are always 4bytes, accidently this was changed to
a void pointer whwqich badly breaks 64bit archs when running on top
of svga3.

Fixes: 2cd80dbd ("drm/vmwgfx: Add basic support for SVGA3")
Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Martin Krastev <krastevm@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210615182336.995192-3-zackr@vmware.com