vfb: fix video mode and line_length being set when loaded

vfb: fix video mode and line_length being set when loaded

Currently, when loading the vfb module, the newly created fbdev
has a line_length of 0, and its video mode would be PSEUDOCOLOR
regardless of color depth. (The former could be worked around by
calling the FBIOPUT_VSCREENINFO ioctl with having the FBACTIVIATE_FORCE
flag set.) This patch automatically sets the line_length correctly,
and the video mode is derived from the bit depth now as well.

Thanks to Geert Uytterhoeven for confirming the bug and helping me with
the patch.

Output of `fbset -i' before the patch:
mode "1366x768-60"
    # D: 72.432 MHz, H: 47.403 kHz, V: 60.004 Hz
    geometry 1366 768 1366 768 32
    timings 13806 120 10 14 3 32 5
    rgba 8/0,8/8,8/16,8/24
endmode

Frame buffer device information:
    Name        : Virtual FB
    Address     : 0xffffaa1405d85000
    Size        : 4196352
    Type        : PACKED PIXELS
    Visual      : PSEUDOCOLOR
    XPanStep    : 1
    YPanStep    : 1
    YWrapStep   : 1
    LineLength  : 0                    <-- note this
    Accelerator : No

After:
mode "1366x768-60"
    # D: 72.432 MHz, H: 47.403 kHz, V: 60.004 Hz
    geometry 1366 768 1366 768 32
    timings 13806 120 10 14 3 32 5
    rgba 8/0,8/8,8/16,8/24
endmode

Frame buffer device information:
    Name        : Virtual FB
    Address     : 0xffffaa1405d85000
    Size        : 4196352
    Type        : PACKED PIXELS
    Visual      : TRUECOLOR
    XPanStep    : 1
    YPanStep    : 1
    YWrapStep   : 1
    LineLength  : 5464
    Accelerator : No

Signed-off-by: "Pieter \"PoroCYon\" Sluys" <pcy@national.shitposting.agency>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
[b.zolnierkie: minor fixups]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>