powerpc/watchpoint: Introduce is_ptrace_bp() function

Introduce is_ptrace_bp() function and move the check inside the
function. It will be utilize more in later set of patches.

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/20200514111741.97993-12-ravi.bangoria@linux.ibm.com