drm/drm_dp_helper: Fix spelling issue and add missing description for 'enable_frl_ready_hpd'

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

 drivers/gpu/drm/drm_dp_helper.c:965: warning: Function parameter or member 'color_spc' not described in 'drm_dp_downstream_rgb_to_ycbcr_conversion'
 drivers/gpu/drm/drm_dp_helper.c:965: warning: Excess function parameter 'colorspc' description in 'drm_dp_downstream_rgb_to_ycbcr_conversion'
 drivers/gpu/drm/drm_dp_helper.c:2678: warning: Function parameter or member 'enable_frl_ready_hpd' not described in 'drm_dp_pcon_frl_prepare'

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
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/20210115181601.3432599-14-lee.jones@linaro.org