drm/i915/fb: move intel_fb_align_height() to intel_fb.c

Split out fb related stuff from intel_display.c to intel_fb.c.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7c97d29eeff676b510eafd242e2a6d7c8ed4a3a6.1629721467.git.jani.nikula@intel.com