drm/amdgpu: Fix allocating extra dwords for rings (v2)

Rename extra_dw to extra_bytes and document what it's for.

The value is already used as if it were bytes in vcn_v4_0.c
and in amdgpu_ring_init. Just adjust the dword count in
jpeg_v1_0.c so that it becomes a byte count.

v2:
Rename extra_dw to extra_bytes as discussed during review.

Fixes: c8c1a1d2 ("drm/amdgpu: define and add extra dword for jpeg ring")
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>