powerpc/watchpoint: Disable all available watchpoints when !dawr_force_enable

Instead of disabling only first watchpoint, disable all available
watchpoints while clearing dawr_force_enable.

Callback function is used only for disabling watchpoint, rename it
to disable_dawrs_cb(). And null_brk parameter is not really required
while disabling watchpoint, remove it.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Michael Neuling <mikey@neuling.org>
Link: https://lore.kernel.org/r/20200514111741.97993-9-ravi.bangoria@linux.ibm.com