drm/framebuffer: Use actual bpp for DRM_IOCTL_MODE_GETFB

When userspace queries the properties of a frame buffer, the number of
bits per pixel is derived from the deprecated drm_format_info.cpp[]
field, which does not take into account block sizes.

Fix this by using the actual number of bits per pixel instead.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/b612780feef200b7f914a674db28ed227e7cdc64.1657294931.git.geert@linux-m68k.org