cg3: add explicit ram_addr_t cast to scanline page variable

cg3: add explicit ram_addr_t cast to scanline page variable

Coverity warns that multiplying two 32-bit values gives a 32-bit result which
is assigned to a 64-bit variable. Add an explicit ram_addr_t cast to silence
the warning.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>