drm/amdgpu/gfx9: Fix incorrect firmware size calculation

[WHY]
The memcpy() function copies n bytes from memory area src to memory area
dest. So specify the firmware size in bytes.

[How]
Correct the calculation.

Acked-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Lei Guo <raykwok1150@163.com>
Reviewed-by: Junwei Zhang <zjunweihit@163.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>