drm/etnaviv: rename pipe_reg_read(..)

pipe_reg_read(..) iterates over all pixel pipes, selects a perf counter
register and sums the actual perf counter value. Rename the function
to reflect more what it is actual doing.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>