drm/fb-helper: Round up bits_per_pixel if possible

When userspace requests a video mode parameter value that is not
supported, frame buffer device drivers should round it up to a supported
value, if possible, instead of just rejecting it.  This allows
applications to quickly scan for supported video modes.

Currently this rule is not followed for the number of bits per pixel,
causing e.g. "fbset -depth N" to fail, if N is smaller than the current
number of bits per pixel.

Fix this by returning an error only if bits per pixel is too large, and
setting it to the current value otherwise.

See also Documentation/fb/framebuffer.rst, Section 2 (Programmer's View
of /dev/fb*").

Fixes: 865afb11 ("drm/fb-helper: reject any changes to the fbdev")
Cc: stable@vger.kernel.org
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191230132734.4538-1-geert+renesas@glider.be