drm/vmwgfx: Fix order of operation

mode->hdisplay * (var->bits_per_pixel + 7) gets evaluated before
the division, potentially making the pitch larger than it should
be.

Since the original intention is to do a div-round-up, just use
the macro instead.

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