drm/amd/display: Align macro name as per DP spec

[Why]:
Aligh with DP spec wanted to follow same naming convention.

[How]:
Changed the macro name of the dpcd address used for getting requested
test-pattern.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316103759.12867-2-animesh.manna@intel.com