drm/pl111: Fix FB depth on IMPD-1 framebuffer

drm/pl111: Fix FB depth on IMPD-1 framebuffer

The last argument to the function drm_fbdev_dma_setup() was
changed from desired BPP to desired depth.

In our case the desired depth was 15 but BPP was 16, so we
specified 16 as BPP and we relied on the FB emulation core to
select a format with a suitable depth for the limited bandwidth
and end up with e.g. XRGB1555 like in the past:

[drm] Initialized pl111 1.0.0 20170317 for c1000000.display on minor 0
drm-clcd-pl111 c1000000.display: [drm] requested bpp 16, scaled depth down to 15
drm-clcd-pl111 c1000000.display: enable IM-PD1 CLCD connectors
Console: switching to colour frame buffer device 80x30
drm-clcd-pl111 c1000000.display: [drm] fb0: pl111drmfb frame buffer device

However the current code will fail at that:

[drm] Initialized pl111 1.0.0 20170317 for c1000000.display on minor 0
drm-clcd-pl111 c1000000.display: [drm] bpp/depth value of 16/16 not supported
drm-clcd-pl111 c1000000.display: [drm] No compatible format found
drm-clcd-pl111 c1000000.display: [drm] *ERROR* fbdev: Failed to setup generic emulation (ret=-12)

Fix this by passing the desired depth of 15 for the IM/PD-1 display
instead of 16 to drm_fbdev_dma_setup().

The desired depth is however in turn used for bandwidth limiting
calculations and that was done with a simple / integer division,
whereas we now have to modify that to use DIV_ROUND_UP() so that
we get DIV_ROUND_UP(15, 2) = 2 not 15/2 = 1.

After this the display works again on the Integrator/AP IM/PD-1.

Cc: Emma Anholt <emma@anholt.net>
Cc: stable@vger.kernel.org
Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 37c90d589dc0 ("drm/fb-helper: Fix single-probe color-format selection")
Link: https://lore.kernel.org/dri-devel/20230102112927.26565-1-tzimmermann@suse.de/
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230515092943.1401558-1-linus.walleij@linaro.org