drm/i915/display: Add HDR Capability detection for LSPCON

LSPCON firmware exposes HDR capability through LPCON_CAPABILITIES
DPCD register. LSPCON implementations capable of supporting
HDR set HDR_CAPABILITY bit in LSPCON_CAPABILITIES to 1. This patch
reads the same, detects the HDR capability and adds this to
intel_lspcon struct.

v2: Addressed Jani Nikula's review comment and fixed the HDR
    capability detection logic

v3: Deferred HDR detection from lspcon_init (Ville)

v4: Addressed Ville's minor review comments, added his RB.

Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130204738.2443-2-uma.shankar@intel.com