vmware_vga: Return a value for FB_SIZE before the device is enabled

vmware_vga: Return a value for FB_SIZE before the device is enabled

According to the documentation drivers using this device should read
FB_SIZE before enabling the device to know what memory to map. This
would not work if we return 0 before enabled. The docs also mention
reading SVGA_REG_DEPTH but not writing it. (Only SVGA_REG_BITS_PER_PIXEL
can be written but we don't really support that either.)

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>