drm/gma500: Remove struct psb_gem_object.npage

Calculate the number of pages in the BO's backing storage from
the size. Remove the npage field.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220308195222.13471-2-tzimmermann@suse.de