drm/gma500: correct kerneldoc

Correct kerneldoc (remove wrong /** marker and adjust function name) to
fix W=1 warnings:

  drivers/gpu/drm/gma500/cdv_intel_lvds.c:27: warning:
    expecting prototype for LVDS I2C backlight control macros(). Prototype was for BRIGHTNESS_MAX_LEVEL() instead

  drivers/gpu/drm/gma500/intel_gmbus.c:386: warning:
    expecting prototype for intel_gmbus_setup(). Prototype was for gma_intel_setup_gmbus() instead

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419081807.68000-1-krzysztof.kozlowski@canonical.com