drm: Update drm_fbdev_cma_init documentation

Commit be7f735cd5ea ("drm: Rely on mode_config data for fb_helper
initialization") dropped the num_crtc argument.  Update the
documentation to reflect that and prevent the kernel-doc warnings below:

./drivers/gpu/drm/drm_fb_cma_helper.c:557: warning: Excess function parameter 'num_crtc' description in 'drm_fbdev_cma_init'
./drivers/gpu/drm/drm_fb_cma_helper.c:558: warning: Excess function parameter 'num_crtc' description in 'drm_fbdev_cma_init'

Fixes: be7f735cd5ea ("drm: Rely on mode_config data for fb_helper initialization")
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/87o9xkvn2m.fsf@dilma.collabora.co.uk