drm/i915/skl: Extract tile height code into a helper function

It will be used in a later patch and also convert all height parameters
from int to unsigned int.

v2: Rebased for fb modifiers.
v3: Fixed v2 rebase.
v4:
   * Height should be unsigned int.
   * Make it take pixel_format for consistency and simplicity.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com> (v1)
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> (v4)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>