sysfb: Enable boot time VESA graphic mode selection

Since switch to simplefb/simpledrm VESA graphic mode selection with vga=
kernel parameter is no longer available with legacy BIOS.

The x86 realmode boot code enables the VESA graphic modes when option
FB_BOOT_VESA_SUPPORT is enabled.

This option is selected by vesafb but not simplefb/simpledrm.

To enable use of VESA modes with simplefb in legacy BIOS boot mode drop
dependency of BOOT_VESA_SUPPORT on FB, also drop the FB_ prefix. Select
the option from sysfb rather than the drivers that depend on it.

The BOOT_VESA_SUPPORT is not specific to framebuffer but rather to x86
platform, move it from fbdev to x86 Kconfig.

Fixes: e3263ab3 ("x86: provide platform-devices for boot-framebuffers")
Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/948c39940a4e99f5b43bdbcbe537faae71a43e1d.1645822213.git.msuchanek@suse.de