drm/i915: Fix bounds check in intel_get_shared_dpll_id()

Fix an off-by-one error in the upper-bound check and while at it clear
up a bit the function.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226203455.23032-2-imre.deak@intel.com