Mark the palette2pixelformat lookup table as const

pixelformat is unsigned, adjust the palette2pixelformat table and
pixelformat_to_palette()

palette_to_pixelformat() is a pure function

pixelformat_to_palette() is a const function
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>