PM / devfreq: bimc-bwmon: set a floor_mbps for irq threshold

Interrupt storm happens when bwmon is enabled for GPU. This is mainly
due to constant low traffic observed with GPU while doing memory
read/write. So as the data rates read from counters are low and so the
threshold set for triggering the interrupt also set as low, which in
turn causes huge number of interrupts. Avoid this by setting a minimum
floor for the irq threshold.

Change-Id: I190fad5108bc24afcb67bec5809485380ee3662e
Signed-off-by: Hanumath Prasad <hpprasad@codeaurora.org>