block: allow REQ_PM requests even when the device is suspended

Some times even though the block device is suspended by the block layer
the low-level driver might want to queue the PM requests to the device.
Allow such requests to get peeked as the blk_pm_add_request() has already
added it to the I/O scheduler otherwise the request would be forever stuck
in the I/O scheduler without being fetched by the driver.

Change-Id: I92fd234cfc22a600057e5e69a9e028b97fa9b53a
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>