drm/amd: Fix a probing order problem on SDMA 2.4

commit 751e293f ("drm/amd: Move microcode init from sw_init to
early_init for SDMA v2.4") made a fateful mistake in
`adev->sdma.num_instances` wasn't declared when sdma_v2_4_init_microcode()
was run. This caused probing to fail.

Move the declaration to right before sdma_v2_4_init_microcode().

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3043
Fixes: 751e293f

 ("drm/amd: Move microcode init from sw_init to early_init for SDMA v2.4")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>