Kernel: Only use multiboot framebuffer details if present

We should only look at the framebuffer structure members if the
MULTIBOOT_INFO_FRAMEBUFFER_INFO bit is set in the flags field.

Also add some logging if we ignored the fbdev command line argument
due to either not having a framebuffer provided by the bootloader, or
because we don't support the framebuffer format.

Kernel/Multiboot.h

@@ -46,6 +46,8 @@ struct multiboot_mmap_entry {
 } __attribute__((packed));
 typedef struct multiboot_mmap_entry multiboot_memory_map_t;
 
+#define MULTIBOOT_INFO_FRAMEBUFFER_INFO (1 << 12)
+
 struct multiboot_info {
     // Multiboot info version number.
     u32 flags;