vgacon: cache vc_cell_height in vgacon_cursor()

There are many places c->vc_cell_height is used in the code of
vgacon_cursor(). Caching the value to a local variable makes the code
much easier to follow.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Helge Deller <deller@gmx.de>