drm/amdgpu: Initialize data to NULL in imu_v12_0_program_rlc_ram()

After a recent change in clang to expose uninitialized warnings from
const variables and pointers [1], there is a warning in
imu_v12_0_program_rlc_ram() because data is passed uninitialized to
program_imu_rlc_ram():

  drivers/gpu/drm/amd/amdgpu/imu_v12_0.c:374:30: error: variable 'data' is uninitialized when used here [-Werror,-Wuninitialized]
    374 |                         program_imu_rlc_ram(adev, data, (const u32)size);
        |                                                   ^~~~

As this warning happens early in clang's frontend, it does not realize
that due to the assignment of r to -EINVAL, program_imu_rlc_ram() is
never actually called, and even if it were, data would not be
dereferenced because size is 0.

Just initialize data to NULL to silence the warning, as the commit that
added program_imu_rlc_ram() mentioned it would eventually be used over
the old method, at which point data can be properly initialized and
used.

Cc: stable@vger.kernel.org
Closes: https://github.com/ClangBuiltLinux/linux/issues/2107
Fixes: 56159fff ("drm/amdgpu: use new method to program rlc ram")
Link: https://github.com/llvm/llvm-project/commit/2464313eef01c5b1edf0eccf57a32cdee01472c7

 [1]
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>