drm/i915/display: clean up intel_PLL_is_valid()

Drop useless macro hiding the return. Fix superfluous whitespace. Rename
function to all lowercase.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/307c9f87cb2fbd5d2d67ec6adcde7ab669c2b93f.1584714939.git.jani.nikula@intel.com