drm: Add HLG EOTF

ADD HLG EOTF to the list of EOTF transfer functions supported.
Hybrid Log-Gamma (HLG) is a high dynamic range (HDR) standard.
HLG defines a nonlinear transfer function in which the lower
half of the signal values use a gamma curve and the upper half
of the signal values use a logarithmic curve.

v2: Rebase

v3: Fixed a warning message

v4: Addressed Shashank's review comments

v5: Addressed Jonas Karlman's review comment and dropped the i915
tag from header.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1558015817-12025-8-git-send-email-uma.shankar@intel.com