drm/tilcdc: fix LCD pixel clock setting

The tilcdc_pclk_diff() compares the requested pixel clock rate to the
real one, so passing it clk_rate instead of clk_rate / clkdiv caused
it to fail even if the clk_rate was properly set. Adding the
real_pclk_rate variable makes the code more readable.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Reviewed-by: Jyri Sarha <jyri.sarha@iki.fi>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Jyri Sarha <jyri.sarha@iki.fi>
Link: https://patchwork.freedesktop.org/patch/msgid/20210322213337.26667-3-dariobin@libero.it