Assuming that your country is the US, the AP105 does have DFS support (AOS 6.3.1.7)
(192.168.1.3) #show ap allowed-channels country-code US ap-type ap-105
Allowed Channels for AP Type ap-105 Country Code "US" Country "United States"
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PHY Type Allowed Channels
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802.11g (indoor) 1 2 3 4 5 6 7 8 9 10 11
802.11a (indoor) 36 40 44 48 52 56 60 64 100 104 108 112 116 132 136 140 149 153 157 161 165
802.11g (outdoor) 1 2 3 4 5 6 7 8 9 10 11
802.11a (outdoor) 52 56 60 64 100 104 108 112 116 132 136 140 149 153 157 161 165
802.11g 40MHz (indoor) 1-5 2-6 3-7 4-8 5-9 6-10 7-11
802.11a 40MHz (indoor) 36-40 44-48 52-56 60-64 100-104 108-112 132-136 149-153 157-161
802.11g 40MHz (outdoor) 1-5 2-6 3-7 4-8 5-9 6-10 7-11
802.11a 40MHz (outdoor) 52-56 60-64 100-104 108-112 132-136 149-153 157-161
802.11a (DFS) 52 56 60 64 100 104 108 112 116 132 136 140
With regards to client support, there are quite a few like Motorola phones for example that do not support DFS. For every client that does not support DFS you would be creating a "coverage hole" or making the 2.4ghz side of the access point more attractive which would sort of defeat the purpose of having 5ghz rates in the first place. If you have a high density environment and only run 20mhz channels with no DFS, you would still have 9 channels to deal with, which in many environments is very good. Organizations that have full control over the majorityof their clients that they desire to do DFS have a better chance of success in a setup with DFS.