drm/msm: support optional messaging in event log

This patch updates the event logging facility to allow
for more fine grained control of the debug messages that
are recorded by the event log. A new 'VERBOSE' macro is
provided to specify messages that may be optionally omitted,
and a debugfs entry is provided to specify a logging filter
based on partial function names. If a filter is specified,
only messages from functions containing any of the filter
keys as part of their name will be logged.

The filtering may be disabled by echoing an empty string
to the debugfs file.

CRs-Fixed: 2023394
Change-Id: Id1476b7f09839e46ef6122837b2f11372fd6a05e
Signed-off-by: Clarence Ip <cip@codeaurora.org>