media: venus: Introduce accessors for remapped hfi_buffer_reqs members

Currently we have macros to access these, but they don't provide a
way to override the remapped fields. Replace the macros with actual
get/set pairs to fix that.

Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Stanimir Varbanov <stanimir.k.varbanov@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>