drm/amdgpu: add missing size check in amdgpu_debugfs_gprwave_read()

Avoid a possible buffer overflow if size is larger than 4K.

Reviewed-by: Yang Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit f5d873f5)
Cc: stable@vger.kernel.org