drm: make is_lspcon_adaptor static

Fixes sparse warning:

drivers/gpu/drm/drm_dp_dual_mode_helper.c:151:6: warning: symbol
'is_lspcon_adaptor' was not declared. Should it be static?

Fixes: 056996b95686 ("drm: Helper for lspcon in drm_dp_dual_mode")
Cc: Rodrigo Vivi <[e-mail]>
Cc: Shashank Sharma <[e-mail]>
Cc: <[e-mail]>
Reviewed-by: Shashank Sharma <[e-mail]>
Signed-off-by: Jani Nikula <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_dp_dual_mode_helper.c

···
 			      DP_DUAL_MODE_REV_TYPE2);
 }
 
-bool is_lspcon_adaptor(const char hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN],
-	const uint8_t adaptor_id)
+static bool is_lspcon_adaptor(const char hdmi_id[DP_DUAL_MODE_HDMI_ID_LEN],
+			      const uint8_t adaptor_id)
 {
 	return is_hdmi_adaptor(hdmi_id) &&
 		(adaptor_id == (DP_DUAL_MODE_TYPE_TYPE2 |