tcx: Fix 24-bit display mode

Commit d08151bf (conversion of tcx to the memory API) broke the 24-bit mode of
the tcx display adapter by accidentally passing in the final address of the
dirty region to memory_region_reset_dirty() instead of its size.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>