drm/edid: Cleanup kerneldoc

This fixes up a couple of inconsistencies with kerneldoc comments for
the EDID related functions. Functions that return a value now use the
"Return" section consistently. For some exported symbols the kernel-doc
comments were not in the appropriate format and therefore not included
in the generated documentation. Others had no kernel-doc at all.

While at it, fix a couple of whitespace issues and capitalize common
abbreviations (i2c -> I2C, edid -> EDID, ...).
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>