drm/amdgpu: drop redundant W=1 warnings from Makefile

Since commit a61ddb43 ("drm: enable (most) W=1 warnings by default
across the subsystem"), most of the extra warnings in the driver
Makefile are redundant. Remove them.

Note that -Wmissing-declarations and -Wmissing-prototypes are always
enabled by default in scripts/Makefile.extrawarn.

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>