drm/i915/display: Eliminate IS_GEN9_{BC,LP}

Now that we've eliminated INTEL_GEN(), IS_GEN_RANGE(), etc. from the
display code, we should also kill off our use of the IS_GEN9_* macros
too.  We'll do the conversion manually this time instead of using
Coccinelle since the most logical substitution can depend heavily on the
code context, and sometimes we can keep the code simpler if we make
additional adjustments such as swapping the order of if/else arms.

v2:
 - Restore a lost negation in intel_pll_is_valid().

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210407203945.1432531-1-matthew.d.roper@intel.com