drm/amdgpu: Update the variable name to dma_buf

Instead of fixing the warning for missing variable
its better to update the variable name to match
with the style followed in the code.

This will fix the below mentioned warning:
warning: Function parameter or struct member 'dbuf' not described in 'amdgpu_bo_create_isp_user'
warning: Excess function parameter 'dma_buf' description in 'amdgpu_bo_create_isp_user'

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>