firmware: Factor out the paged buffer handling code

This is merely a preparation for the upcoming compressed firmware
support and no functional changes.  It moves the code to handle the
paged buffer allocation and mapping out of fallback.c into the main
code, so that they can be used commonly.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>