arch/powerpc/perf: Update get_mem_data_src function to use saved values of sier and mmcra regs

During performance monitor interrupt handling, the regs are setup using
perf_read_regs function. Here some of the pt_regs fields is overloaded.
Samples Instruction Event Register (SIER) is loaded into pt_regs,
overloading regs->dar. And regs->dsisr to store MMCRA (Monitor Mode
Control Register A) so that we only need to read these once on each
interrupt.

Update the isa207_get_mem_data_src function to use regs->dar instead of
reading from SPRN_SIER again. Also use regs->dsisr to read the mmcra
value

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250121131621.39054-2-atrajeev@linux.vnet.ibm.com