drm: Remove pointless '-' characters from drm_fb_helper documentation

/**

 * struct drm_fb_helper_funcs - driver callbacks for the fbdev emulation library

 * @gamma_set: - Set the given gamma lut register on the given crtc.

 * @gamma_get: - Read the given gamma lut register on the given crtc, used to

 * @gamma_set: Set the given gamma lut register on the given crtc.

 * @gamma_get: Read the given gamma lut register on the given crtc, used to

 *             save the current lut when force-restoring the fbdev for e.g.

 *             kdbg.

 * @fb_probe: - Driver callback to allocate and initialize the fbdev info

 * @fb_probe: Driver callback to allocate and initialize the fbdev info

 *            structure. Futhermore it also needs to allocate the drm

 *            framebuffer used to back the fbdev.

 * @initial_config: - Setup an initial fbdev display configuration

 * @initial_config: Setup an initial fbdev display configuration

 *

 * Driver callbacks used by the fbdev emulation helper library.

 */