snprintf can truncate the output fw filename if the isp ucode_prefix
exceeds 29 characters. Knowing ISP ucode_prefix is in the format
isp_x_x_x, limiting the size of ucode_prefix[] to 10 characters
to fix the warning.

Fixes the below warning:

   drivers/gpu/drm/amd/amdgpu/amdgpu_isp.c: In function 'isp_early_init':
   drivers/gpu/drm/amd/amdgpu/amdgpu_isp.c:192:58: warning: 'snprintf'
   output may be truncated before the last format character
   [-Wformat-truncation=]
     192 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);
         |                                                          ^
   In function 'isp_load_fw_by_psp',
       inlined from 'isp_early_init' at drivers/gpu/drm/amd/amdgpu/amdgpu_isp.c:218:8:
   drivers/gpu/drm/amd/amdgpu/amdgpu_isp.c:192:9: note: 'snprintf' output between 12 and 41 bytes into a destination of size 40
     192 |         snprintf(fw_name, sizeof(fw_name), "amdgpu/%s.bin", ucode_prefix);
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Pratap Nirujogi <pratap.nirujogi@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>