drm/amd/display: Drop unnecessary `amdgpu` prefix

[Why]
The `drm_*()` print macros will handle including the driver in the print
already.  The extra print of the word `amdgpu` is unnecessary.

[How]
Modify all prints to drop `amdgpu: `.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Wayne Lin <wayne.lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>