drm/i915/glk: Fix degamma lut programming

Fixed the glk degamma lut programming which currently
was hard coding a linear lut all the time, making degamma
block of glk basically a pass through.

Currently degamma lut for glk is assigned as 0 in platform
configuration. Updated the same to 33 as per the hardware
capability. IGT tests for degamma were getting skipped due to
this, spotted by Swati.

ToDo: The current gamma/degamm lut ABI has just 16bit for each
color component. This is not enough for GLK+, since input
precision is increased to 3.16 which will need 19bit entries.

v2: Added Matt's RB.

v3: Changed uint32_t to u32.

v4: Fixed Maarten's review comment

Credits-to: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1549893025-21837-2-git-send-email-uma.shankar@intel.com