drm/msm/sde: add utility functions for kms info

Add utility functions to support creation of ASCII information
text of the form,

"key=value\n"

One or more such information strings may be combined together
and stored within an sde_kms_info structure to be used at a
later time.

Change-Id: I7fccc14548e66ed7c5b24dc129247572209d506b
Signed-off-by: Clarence Ip <cip@codeaurora.org>