drm/i915: Replace some gamma_mode ifs with switches

Since gamma_mode can have more than two values on ilk+
let's use switch statements when interpreting them.

v2: Fix typo (Uma)

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925131656.10022-10-ville.syrjala@linux.intel.com