drm/i915/vlv_dsi: Double pixelclock on read-back for dual-link panels

In intel_dsi_get_config() double the pclk returned by foo_dsi_get_pclk()
for dual-link panels. This fixes the following WARN triggering:

 i915 0000:00:02.0: [drm] *ERROR* [CRTC:51:pipe A] mismatch in pixel_rate (expected 235710, found 118056)
 i915 0000:00:02.0: [drm] *ERROR* [CRTC:51:pipe A] mismatch in hw.pipe_mode.crtc_clock (expected 235710, found 118056)
 i915 0000:00:02.0: [drm] *ERROR* [CRTC:51:pipe A] mismatch in hw.adjusted_mode.crtc_clock (expected 235710, found 118056)
 i915 0000:00:02.0: [drm] *ERROR* [CRTC:51:pipe A] mismatch in port_clock (expected 235710, found 118056)
 ------------[ cut here ]------------
 pipe state doesn't match!
 WARNING: CPU: 3 PID: 136 at drivers/gpu/drm/i915/display/intel_display.c:9125 intel_display_finish_reset+0x1bd3/0x2050 [i915]
 ...

This has been tested on a Xiaomi Mi Pad 2 (with CHT x5-Z8500 SoC) tablet,
with a 1536x2048 dual-link DSI panel.

Note this fix was taken from icl_dsi.c which does the same in
its get_config().

Cc: Tsuchiya Yuto <kitakar@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211024155020.126328-1-hdegoede@redhat.com