drm/gma500/oaktrail_crtc: Demote unworthy kernel-doc headers

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/gma500/oaktrail_crtc.c:184: warning: Function parameter or member 'limit' not described in 'mrst_lvds_find_best_pll'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:184: warning: Function parameter or member 'crtc' not described in 'mrst_lvds_find_best_pll'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:184: warning: Function parameter or member 'target' not described in 'mrst_lvds_find_best_pll'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:184: warning: Function parameter or member 'refclk' not described in 'mrst_lvds_find_best_pll'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:184: warning: Function parameter or member 'best_clock' not described in 'mrst_lvds_find_best_pll'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:215: warning: Function parameter or member 'crtc' not described in 'oaktrail_crtc_dpms'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:215: warning: Function parameter or member 'mode' not described in 'oaktrail_crtc_dpms'
 drivers/gpu/drm/gma500/oaktrail_crtc.c:345: warning: Function parameter or member 'dev' not described in 'oaktrail_panel_fitter_pipe'

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181601.3432599-21-lee.jones@linaro.org