drm/i915: Remove the unused p and m limits for VLV

We never check the p and m limits (which according to comments are
based on someone's guesswork), so just remove them.

VLV2_DPLL_mphy_hsdpll_frequency_table_ww6_rev1p1.xlsm has no p and m
limits listed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>