Stop the IPU before disabling its clock
Clear the REG_CTRL register instead of setting its RST bit

Strangely enough, waiting in a long enough usleep() right after
setting the RST bit would prevent the IPU from outputing an image.

On the other hand, clearing the REG_CTRL register does not have this
surprising behaviour. Since all the registers we use will obtain
new values anyway, it is not absolutely necesary to reset them.