drm/fb_helper: Create wrappers for fb_sys_read/write funcs

Some drm drivers populate their fb_ops with fb_sys_read/write fb sysfs
ops.

Create a drm_fb_helper function that wraps around these calls.

This is part of an effort to prevent drm drivers from calling fbdev
functions directly, in order to make fbdev emulation a top level drm
option.

v3:
- Fix kerneldoc errors

v2:
- Added kerneldocs
- Follow the drm way of aligning of arguments in func definitions
- Remove unnecessary checks for non NULL fb_info

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>