Fix huge MIOpen workspace allocations

[pxl-th]

• Use workspace size suggested from find_conv_algo function.
  Using get_workspace_size function for initial workspace allocation can lead to unncessary huge memory allocations (up to 2.5 GiB). Here's an example of convolution that led to 2.5 GiB workspace allocation.

c = Conv((3, 3), 1280=>1280) |> f16 |> gpu
x = rand(Float32, 32, 32, 1280, 1) |> f16 |> gpu
y = c(x)

Workspace size from find_conv_algo takes only ~15 MiB.

• Refactor allocation code a bit