drm/bridge: tc358768: fix PLL target frequency

Correctly compute the PLL target frequency, the current formula works
correctly only when the input bus width is 24bit, actually to properly
compute the PLL target frequency what is relevant is the bits-per-pixel
on the DSI link.

No regression expected since the DSI format is currently hard-coded as
MIPI_DSI_FMT_RGB888.

Fixes: ff1ca639 ("drm/bridge: Add tc358768 driver")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230427142934.55435-4-francesco@dolcini.it